diff --git a/src/program.ts b/src/program.ts
index adedcf2caf..e3c183a096 100644
--- a/src/program.ts
+++ b/src/program.ts
@@ -1213,11 +1213,9 @@ export class Program extends DiagnosticEmitter {
       let basePrototype = thisPrototype.basePrototype;
       let interfacePrototypes = thisPrototype.interfacePrototypes;
       if (basePrototype) {
-        assert(!interfacePrototypes);
         this.markVirtuals(thisPrototype, basePrototype);
       }
       if (interfacePrototypes) {
-        assert(!basePrototype);
         for (let j = 0, l = interfacePrototypes.length; j < l; ++j) {
           this.markVirtuals(thisPrototype, interfacePrototypes[j]);
         }
diff --git a/tests/compiler/class-implements.json b/tests/compiler/class-implements.json
new file mode 100644
index 0000000000..b1da366ff4
--- /dev/null
+++ b/tests/compiler/class-implements.json
@@ -0,0 +1,5 @@
+{
+  "asc_flags": [
+    "--runtime none"
+  ]
+}
\ No newline at end of file
diff --git a/tests/compiler/class-implements.optimized.wat b/tests/compiler/class-implements.optimized.wat
new file mode 100644
index 0000000000..28eafbd5a4
--- /dev/null
+++ b/tests/compiler/class-implements.optimized.wat
@@ -0,0 +1,112 @@
+(module
+ (type $i32_=>_i32 (func (param i32) (result i32)))
+ (type $none_=>_none (func))
+ (memory $0 1)
+ (data (i32.const 1024) "&\00\00\00\01\00\00\00\01\00\00\00&\00\00\00c\00l\00a\00s\00s\00-\00i\00m\00p\00l\00e\00m\00e\00n\00t\00s\00.\00t\00s")
+ (global $~lib/rt/stub/offset (mut i32) (i32.const 0))
+ (global $class-implements/A i32 (i32.const 3))
+ (global $class-implements/C i32 (i32.const 5))
+ (export "memory" (memory $0))
+ (export "A" (global $class-implements/A))
+ (export "A#foo" (func $class-implements/A#foo))
+ (export "A#constructor" (func $class-implements/A#constructor))
+ (export "C" (global $class-implements/C))
+ (export "C#foo" (func $class-implements/C#foo))
+ (start $~start)
+ (func $~lib/rt/stub/__alloc (param $0 i32) (result i32)
+  (local $1 i32)
+  (local $2 i32)
+  (local $3 i32)
+  (local $4 i32)
+  global.get $~lib/rt/stub/offset
+  i32.const 16
+  i32.add
+  local.tee $3
+  i32.const 16
+  i32.add
+  local.tee $1
+  memory.size
+  local.tee $4
+  i32.const 16
+  i32.shl
+  local.tee $2
+  i32.gt_u
+  if
+   local.get $4
+   local.get $1
+   local.get $2
+   i32.sub
+   i32.const 65535
+   i32.add
+   i32.const -65536
+   i32.and
+   i32.const 16
+   i32.shr_u
+   local.tee $2
+   local.get $4
+   local.get $2
+   i32.gt_s
+   select
+   memory.grow
+   i32.const 0
+   i32.lt_s
+   if
+    local.get $2
+    memory.grow
+    i32.const 0
+    i32.lt_s
+    if
+     unreachable
+    end
+   end
+  end
+  local.get $1
+  global.set $~lib/rt/stub/offset
+  local.get $3
+  i32.const 16
+  i32.sub
+  local.tee $1
+  i32.const 16
+  i32.store
+  local.get $1
+  i32.const 1
+  i32.store offset=4
+  local.get $1
+  local.get $0
+  i32.store offset=8
+  local.get $1
+  i32.const 0
+  i32.store offset=12
+  local.get $3
+ )
+ (func $class-implements/A#constructor (param $0 i32) (result i32)
+  local.get $0
+  if (result i32)
+   local.get $0
+  else
+   i32.const 3
+   call $~lib/rt/stub/__alloc
+  end
+ )
+ (func $class-implements/A#foo (param $0 i32) (result i32)
+  i32.const 1
+ )
+ (func $class-implements/C#foo (param $0 i32) (result i32)
+  i32.const 2
+ )
+ (func $~start
+  i32.const 1088
+  global.set $~lib/rt/stub/offset
+  i32.const 0
+  call $class-implements/A#constructor
+  drop
+  i32.const 5
+  call $~lib/rt/stub/__alloc
+  i32.eqz
+  if
+   i32.const 6
+   call $~lib/rt/stub/__alloc
+   drop
+  end
+ )
+)
diff --git a/tests/compiler/class-implements.ts b/tests/compiler/class-implements.ts
new file mode 100644
index 0000000000..d284e80068
--- /dev/null
+++ b/tests/compiler/class-implements.ts
@@ -0,0 +1,20 @@
+interface I {
+  foo(): i32;
+}
+
+export class A implements I {
+  foo(): i32 { return 1; }
+}
+
+var a = new A();
+assert(a.foo() === 1);
+
+class B {
+}
+
+export class C extends B implements I {
+  foo(): i32 { return 2; }
+}
+
+var c = new C();
+assert(c.foo() === 2);
diff --git a/tests/compiler/class-implements.untouched.wat b/tests/compiler/class-implements.untouched.wat
new file mode 100644
index 0000000000..b7ab1b4de0
--- /dev/null
+++ b/tests/compiler/class-implements.untouched.wat
@@ -0,0 +1,229 @@
+(module
+ (type $i32_=>_i32 (func (param i32) (result i32)))
+ (type $none_=>_none (func))
+ (type $i32_=>_none (func (param i32)))
+ (type $i32_i32_i32_i32_=>_none (func (param i32 i32 i32 i32)))
+ (type $i32_i32_=>_i32 (func (param i32 i32) (result i32)))
+ (import "env" "abort" (func $~lib/builtins/abort (param i32 i32 i32 i32)))
+ (memory $0 1)
+ (data (i32.const 16) "&\00\00\00\01\00\00\00\01\00\00\00&\00\00\00c\00l\00a\00s\00s\00-\00i\00m\00p\00l\00e\00m\00e\00n\00t\00s\00.\00t\00s\00")
+ (table $0 1 funcref)
+ (global $~lib/rt/stub/startOffset (mut i32) (i32.const 0))
+ (global $~lib/rt/stub/offset (mut i32) (i32.const 0))
+ (global $class-implements/a (mut i32) (i32.const 0))
+ (global $class-implements/c (mut i32) (i32.const 0))
+ (global $~lib/heap/__heap_base i32 (i32.const 72))
+ (global $class-implements/A i32 (i32.const 3))
+ (global $class-implements/C i32 (i32.const 5))
+ (export "memory" (memory $0))
+ (export "A" (global $class-implements/A))
+ (export "A#foo" (func $class-implements/A#foo))
+ (export "A#constructor" (func $class-implements/A#constructor))
+ (export "C" (global $class-implements/C))
+ (export "C#foo" (func $class-implements/C#foo))
+ (start $~start)
+ (func $~lib/rt/stub/maybeGrowMemory (param $0 i32)
+  (local $1 i32)
+  (local $2 i32)
+  (local $3 i32)
+  (local $4 i32)
+  (local $5 i32)
+  memory.size
+  local.set $1
+  local.get $1
+  i32.const 16
+  i32.shl
+  local.set $2
+  local.get $0
+  local.get $2
+  i32.gt_u
+  if
+   local.get $0
+   local.get $2
+   i32.sub
+   i32.const 65535
+   i32.add
+   i32.const 65535
+   i32.const -1
+   i32.xor
+   i32.and
+   i32.const 16
+   i32.shr_u
+   local.set $3
+   local.get $1
+   local.tee $4
+   local.get $3
+   local.tee $5
+   local.get $4
+   local.get $5
+   i32.gt_s
+   select
+   local.set $4
+   local.get $4
+   memory.grow
+   i32.const 0
+   i32.lt_s
+   if
+    local.get $3
+    memory.grow
+    i32.const 0
+    i32.lt_s
+    if
+     unreachable
+    end
+   end
+  end
+  local.get $0
+  global.set $~lib/rt/stub/offset
+ )
+ (func $~lib/rt/stub/__alloc (param $0 i32) (param $1 i32) (result i32)
+  (local $2 i32)
+  (local $3 i32)
+  (local $4 i32)
+  (local $5 i32)
+  (local $6 i32)
+  local.get $0
+  i32.const 1073741808
+  i32.gt_u
+  if
+   unreachable
+  end
+  global.get $~lib/rt/stub/offset
+  i32.const 16
+  i32.add
+  local.set $2
+  local.get $0
+  i32.const 15
+  i32.add
+  i32.const 15
+  i32.const -1
+  i32.xor
+  i32.and
+  local.tee $3
+  i32.const 16
+  local.tee $4
+  local.get $3
+  local.get $4
+  i32.gt_u
+  select
+  local.set $5
+  local.get $2
+  local.get $5
+  i32.add
+  call $~lib/rt/stub/maybeGrowMemory
+  local.get $2
+  i32.const 16
+  i32.sub
+  local.set $6
+  local.get $6
+  local.get $5
+  i32.store
+  i32.const 1
+  drop
+  local.get $6
+  i32.const 1
+  i32.store offset=4
+  local.get $6
+  local.get $1
+  i32.store offset=8
+  local.get $6
+  local.get $0
+  i32.store offset=12
+  local.get $2
+ )
+ (func $~lib/rt/stub/__retain (param $0 i32) (result i32)
+  local.get $0
+ )
+ (func $class-implements/A#constructor (param $0 i32) (result i32)
+  local.get $0
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 3
+   call $~lib/rt/stub/__alloc
+   call $~lib/rt/stub/__retain
+   local.set $0
+  end
+  local.get $0
+ )
+ (func $class-implements/A#foo (param $0 i32) (result i32)
+  i32.const 1
+ )
+ (func $class-implements/B#constructor (param $0 i32) (result i32)
+  local.get $0
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 6
+   call $~lib/rt/stub/__alloc
+   call $~lib/rt/stub/__retain
+   local.set $0
+  end
+  local.get $0
+ )
+ (func $class-implements/C#constructor (param $0 i32) (result i32)
+  local.get $0
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 5
+   call $~lib/rt/stub/__alloc
+   call $~lib/rt/stub/__retain
+   local.set $0
+  end
+  local.get $0
+  call $class-implements/B#constructor
+  local.set $0
+  local.get $0
+ )
+ (func $class-implements/C#foo (param $0 i32) (result i32)
+  i32.const 2
+ )
+ (func $start:class-implements
+  global.get $~lib/heap/__heap_base
+  i32.const 15
+  i32.add
+  i32.const 15
+  i32.const -1
+  i32.xor
+  i32.and
+  global.set $~lib/rt/stub/startOffset
+  global.get $~lib/rt/stub/startOffset
+  global.set $~lib/rt/stub/offset
+  i32.const 0
+  call $class-implements/A#constructor
+  global.set $class-implements/a
+  global.get $class-implements/a
+  call $class-implements/A#foo
+  i32.const 1
+  i32.eq
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 32
+   i32.const 10
+   i32.const 1
+   call $~lib/builtins/abort
+   unreachable
+  end
+  i32.const 0
+  call $class-implements/C#constructor
+  global.set $class-implements/c
+  global.get $class-implements/c
+  call $class-implements/C#foo
+  i32.const 2
+  i32.eq
+  i32.eqz
+  if
+   i32.const 0
+   i32.const 32
+   i32.const 20
+   i32.const 1
+   call $~lib/builtins/abort
+   unreachable
+  end
+ )
+ (func $~start
+  call $start:class-implements
+ )
+)