diff --git a/src/compiler/checker.ts b/src/compiler/checker.ts
index c31732cc24ac9..06395d935b7df 100644
--- a/src/compiler/checker.ts
+++ b/src/compiler/checker.ts
@@ -20573,10 +20573,15 @@ namespace ts {
                 }
                 propType = getConstraintForLocation(getTypeOfSymbol(prop), node);
             }
+            return getFlowTypeOfAccessExpression(node, prop, propType, right);
+        }
+
+        function getFlowTypeOfAccessExpression(node: ElementAccessExpression | PropertyAccessExpression | QualifiedName, prop: Symbol | undefined, propType: Type, errorNode: Node) {
             // Only compute control flow type if this is a property access expression that isn't an
             // assignment target, and the referenced property was declared as a variable, property,
             // accessor, or optional method.
-            if (node.kind !== SyntaxKind.PropertyAccessExpression ||
+            const assignmentKind = getAssignmentTargetKind(node);
+            if (node.kind !== SyntaxKind.ElementAccessExpression && node.kind !== SyntaxKind.PropertyAccessExpression ||
                 assignmentKind === AssignmentKind.Definite ||
                 prop && !(prop.flags & (SymbolFlags.Variable | SymbolFlags.Property | SymbolFlags.Accessor)) && !(prop.flags & SymbolFlags.Method && propType.flags & TypeFlags.Union)) {
                 return propType;
@@ -20586,7 +20591,7 @@ namespace ts {
             // and if we are in a constructor of the same class as the property declaration, assume that
             // the property is uninitialized at the top of the control flow.
             let assumeUninitialized = false;
-            if (strictNullChecks && strictPropertyInitialization && left.kind === SyntaxKind.ThisKeyword) {
+            if (strictNullChecks && strictPropertyInitialization && node.expression.kind === SyntaxKind.ThisKeyword) {
                 const declaration = prop && prop.valueDeclaration;
                 if (declaration && isInstancePropertyWithoutInitializer(declaration)) {
                     const flowContainer = getControlFlowContainer(node);
@@ -20603,7 +20608,7 @@ namespace ts {
             }
             const flowType = getFlowTypeOfReference(node, propType, assumeUninitialized ? getOptionalType(propType) : propType);
             if (assumeUninitialized && !(getFalsyFlags(propType) & TypeFlags.Undefined) && getFalsyFlags(flowType) & TypeFlags.Undefined) {
-                error(right, Diagnostics.Property_0_is_used_before_being_assigned, symbolToString(prop!)); // TODO: GH#18217
+                error(errorNode, Diagnostics.Property_0_is_used_before_being_assigned, symbolToString(prop!)); // TODO: GH#18217
                 // Return the declared type to reduce follow-on errors
                 return propType;
             }
@@ -20960,7 +20965,7 @@ namespace ts {
                 AccessFlags.Writing | (isGenericObjectType(objectType) && !isThisTypeParameter(objectType) ? AccessFlags.NoIndexSignatures : 0) :
                 AccessFlags.None;
             const indexedAccessType = getIndexedAccessTypeOrUndefined(objectType, effectiveIndexType, node, accessFlags) || errorType;
-            return checkIndexedAccessIndexType(indexedAccessType, node);
+            return checkIndexedAccessIndexType(getFlowTypeOfAccessExpression(node, indexedAccessType.symbol, indexedAccessType, indexExpression), node);
         }
 
         function checkThatExpressionIsProperSymbolReference(expression: Expression, expressionType: Type, reportError: boolean): boolean {
diff --git a/tests/baselines/reference/controlFlowElementAccess2.js b/tests/baselines/reference/controlFlowElementAccess2.js
new file mode 100644
index 0000000000000..050fc15b8af1f
--- /dev/null
+++ b/tests/baselines/reference/controlFlowElementAccess2.js
@@ -0,0 +1,25 @@
+//// [controlFlowElementAccess2.ts]
+declare const config: {
+    [key: string]: boolean | { prop: string };
+};
+
+if (typeof config['works'] !== 'boolean') {
+    config.works.prop = 'test'; // ok
+    config['works'].prop = 'test'; // error, config['works']: boolean | { 'prop': string }
+}
+if (typeof config.works !== 'boolean') {
+    config['works'].prop = 'test'; // error, config['works']: boolean | { 'prop': string }
+    config.works.prop = 'test'; // ok
+}
+
+
+//// [controlFlowElementAccess2.js]
+"use strict";
+if (typeof config['works'] !== 'boolean') {
+    config.works.prop = 'test'; // ok
+    config['works'].prop = 'test'; // error, config['works']: boolean | { 'prop': string }
+}
+if (typeof config.works !== 'boolean') {
+    config['works'].prop = 'test'; // error, config['works']: boolean | { 'prop': string }
+    config.works.prop = 'test'; // ok
+}
diff --git a/tests/baselines/reference/controlFlowElementAccess2.symbols b/tests/baselines/reference/controlFlowElementAccess2.symbols
new file mode 100644
index 0000000000000..5cdd890a75973
--- /dev/null
+++ b/tests/baselines/reference/controlFlowElementAccess2.symbols
@@ -0,0 +1,37 @@
+=== tests/cases/conformance/controlFlow/controlFlowElementAccess2.ts ===
+declare const config: {
+>config : Symbol(config, Decl(controlFlowElementAccess2.ts, 0, 13))
+
+    [key: string]: boolean | { prop: string };
+>key : Symbol(key, Decl(controlFlowElementAccess2.ts, 1, 5))
+>prop : Symbol(prop, Decl(controlFlowElementAccess2.ts, 1, 30))
+
+};
+
+if (typeof config['works'] !== 'boolean') {
+>config : Symbol(config, Decl(controlFlowElementAccess2.ts, 0, 13))
+
+    config.works.prop = 'test'; // ok
+>config.works.prop : Symbol(prop, Decl(controlFlowElementAccess2.ts, 1, 30))
+>config : Symbol(config, Decl(controlFlowElementAccess2.ts, 0, 13))
+>prop : Symbol(prop, Decl(controlFlowElementAccess2.ts, 1, 30))
+
+    config['works'].prop = 'test'; // error, config['works']: boolean | { 'prop': string }
+>config['works'].prop : Symbol(prop, Decl(controlFlowElementAccess2.ts, 1, 30))
+>config : Symbol(config, Decl(controlFlowElementAccess2.ts, 0, 13))
+>prop : Symbol(prop, Decl(controlFlowElementAccess2.ts, 1, 30))
+}
+if (typeof config.works !== 'boolean') {
+>config : Symbol(config, Decl(controlFlowElementAccess2.ts, 0, 13))
+
+    config['works'].prop = 'test'; // error, config['works']: boolean | { 'prop': string }
+>config['works'].prop : Symbol(prop, Decl(controlFlowElementAccess2.ts, 1, 30))
+>config : Symbol(config, Decl(controlFlowElementAccess2.ts, 0, 13))
+>prop : Symbol(prop, Decl(controlFlowElementAccess2.ts, 1, 30))
+
+    config.works.prop = 'test'; // ok
+>config.works.prop : Symbol(prop, Decl(controlFlowElementAccess2.ts, 1, 30))
+>config : Symbol(config, Decl(controlFlowElementAccess2.ts, 0, 13))
+>prop : Symbol(prop, Decl(controlFlowElementAccess2.ts, 1, 30))
+}
+
diff --git a/tests/baselines/reference/controlFlowElementAccess2.types b/tests/baselines/reference/controlFlowElementAccess2.types
new file mode 100644
index 0000000000000..65dca4de50226
--- /dev/null
+++ b/tests/baselines/reference/controlFlowElementAccess2.types
@@ -0,0 +1,63 @@
+=== tests/cases/conformance/controlFlow/controlFlowElementAccess2.ts ===
+declare const config: {
+>config : { [key: string]: boolean | { prop: string; }; }
+
+    [key: string]: boolean | { prop: string };
+>key : string
+>prop : string
+
+};
+
+if (typeof config['works'] !== 'boolean') {
+>typeof config['works'] !== 'boolean' : boolean
+>typeof config['works'] : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>config['works'] : boolean | { prop: string; }
+>config : { [key: string]: boolean | { prop: string; }; }
+>'works' : "works"
+>'boolean' : "boolean"
+
+    config.works.prop = 'test'; // ok
+>config.works.prop = 'test' : "test"
+>config.works.prop : string
+>config.works : { prop: string; }
+>config : { [key: string]: boolean | { prop: string; }; }
+>works : { prop: string; }
+>prop : string
+>'test' : "test"
+
+    config['works'].prop = 'test'; // error, config['works']: boolean | { 'prop': string }
+>config['works'].prop = 'test' : "test"
+>config['works'].prop : string
+>config['works'] : { prop: string; }
+>config : { [key: string]: boolean | { prop: string; }; }
+>'works' : "works"
+>prop : string
+>'test' : "test"
+}
+if (typeof config.works !== 'boolean') {
+>typeof config.works !== 'boolean' : boolean
+>typeof config.works : "string" | "number" | "bigint" | "boolean" | "symbol" | "undefined" | "object" | "function"
+>config.works : boolean | { prop: string; }
+>config : { [key: string]: boolean | { prop: string; }; }
+>works : boolean | { prop: string; }
+>'boolean' : "boolean"
+
+    config['works'].prop = 'test'; // error, config['works']: boolean | { 'prop': string }
+>config['works'].prop = 'test' : "test"
+>config['works'].prop : string
+>config['works'] : { prop: string; }
+>config : { [key: string]: boolean | { prop: string; }; }
+>'works' : "works"
+>prop : string
+>'test' : "test"
+
+    config.works.prop = 'test'; // ok
+>config.works.prop = 'test' : "test"
+>config.works.prop : string
+>config.works : { prop: string; }
+>config : { [key: string]: boolean | { prop: string; }; }
+>works : { prop: string; }
+>prop : string
+>'test' : "test"
+}
+
diff --git a/tests/cases/conformance/controlFlow/controlFlowElementAccess2.ts b/tests/cases/conformance/controlFlow/controlFlowElementAccess2.ts
new file mode 100644
index 0000000000000..fa0592c973ee3
--- /dev/null
+++ b/tests/cases/conformance/controlFlow/controlFlowElementAccess2.ts
@@ -0,0 +1,13 @@
+// @strict: true
+declare const config: {
+    [key: string]: boolean | { prop: string };
+};
+
+if (typeof config['works'] !== 'boolean') {
+    config.works.prop = 'test'; // ok
+    config['works'].prop = 'test'; // error, config['works']: boolean | { 'prop': string }
+}
+if (typeof config.works !== 'boolean') {
+    config['works'].prop = 'test'; // error, config['works']: boolean | { 'prop': string }
+    config.works.prop = 'test'; // ok
+}