diff --git a/lib/Sema/TypeCheckDecl.cpp b/lib/Sema/TypeCheckDecl.cpp
index f993087abe611..a6270579b6a3c 100644
--- a/lib/Sema/TypeCheckDecl.cpp
+++ b/lib/Sema/TypeCheckDecl.cpp
@@ -4970,6 +4970,13 @@ void TypeChecker::validateExtension(ExtensionDecl *ext) {
   if (!isa<SourceFile>(dc))
     return;
 
+  // If this is not bound to any decls at this point, this extension is in
+  // inactive coditional compilation block. It's not safe to typecheck this
+  // extension. This happens if code completion is triggered in inactive
+  // conditional complation block.
+  if (!ext->alreadyBoundToNominal())
+    return;
+
   // Validate the nominal type declaration being extended.
   validateDecl(nominal);
 
diff --git a/validation-test/IDE/crashers_2_fixed/sr9001.swift b/validation-test/IDE/crashers_2_fixed/sr9001.swift
new file mode 100644
index 0000000000000..a35b02ada948f
--- /dev/null
+++ b/validation-test/IDE/crashers_2_fixed/sr9001.swift
@@ -0,0 +1,20 @@
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=COMPLETE_1 -source-filename=%s
+// RUN: %target-swift-ide-test -code-completion -code-completion-token=COMPLETE_2 -source-filename=%s
+
+struct MyStruct<T> {
+}
+
+protocol MyProto {
+  associatedtype Assoc
+  func foo(x: Assoc) -> Assoc
+}
+
+#if false
+extension MyStruct {
+  #^COMPLETE_1^#
+}
+
+extension MyStruct: MyProto {
+  #^COMPLETE_2^#
+}
+#endif