diff --git a/internal/checker/checker.go b/internal/checker/checker.go
index d8de6b0419..ca40aafc20 100644
--- a/internal/checker/checker.go
+++ b/internal/checker/checker.go
@@ -8053,7 +8053,6 @@ func (c *Checker) checkCallExpression(node *ast.Node, checkMode CheckMode) *Type
 		if !ast.IsDottedName(node.Expression()) {
 			c.error(node.Expression(), diagnostics.Assertions_require_the_call_target_to_be_an_identifier_or_qualified_name)
 		} else if c.getEffectsSignature(node) == nil {
-			c.error(node.Expression(), diagnostics.Assertions_require_every_name_in_the_call_target_to_be_declared_with_an_explicit_type_annotation)
 			diagnostic := c.error(node.Expression(), diagnostics.Assertions_require_every_name_in_the_call_target_to_be_declared_with_an_explicit_type_annotation)
 			c.getTypeOfDottedName(node.Expression(), diagnostic)
 		}
diff --git a/internal/checker/flow.go b/internal/checker/flow.go
index 3ebbf9bf54..6ff6acbf4c 100644
--- a/internal/checker/flow.go
+++ b/internal/checker/flow.go
@@ -2160,7 +2160,17 @@ func (c *Checker) getExplicitTypeOfSymbol(symbol *ast.Symbol, diagnostic *ast.Di
 }
 
 func (c *Checker) isDeclarationWithExplicitTypeAnnotation(node *ast.Node) bool {
-	return (ast.IsVariableDeclaration(node) || ast.IsPropertyDeclaration(node) || ast.IsPropertySignatureDeclaration(node) || ast.IsParameter(node)) && node.Type() != nil
+	return (ast.IsVariableDeclaration(node) || ast.IsPropertyDeclaration(node) || ast.IsPropertySignatureDeclaration(node) || ast.IsParameter(node)) && node.Type() != nil ||
+		c.isExpandoPropertyFunctionWithReturnTypeAnnotation(node)
+}
+
+func (c *Checker) isExpandoPropertyFunctionWithReturnTypeAnnotation(node *ast.Node) bool {
+	if ast.IsBinaryExpression(node) {
+		if expr := node.AsBinaryExpression().Right; ast.IsFunctionLike(expr) && expr.Type() != nil {
+			return true
+		}
+	}
+	return false
 }
 
 func (c *Checker) hasTypePredicateOrNeverReturnType(sig *Signature) bool {
diff --git a/testdata/baselines/reference/compiler/expandoFunctionAsAssertion.symbols b/testdata/baselines/reference/compiler/expandoFunctionAsAssertion.symbols
new file mode 100644
index 0000000000..a046544871
--- /dev/null
+++ b/testdata/baselines/reference/compiler/expandoFunctionAsAssertion.symbols
@@ -0,0 +1,27 @@
+//// [tests/cases/compiler/expandoFunctionAsAssertion.ts] ////
+
+=== expandoFunctionAsAssertion.ts ===
+function example() {}
+>example : Symbol(example, Decl(expandoFunctionAsAssertion.ts, 0, 0))
+
+example.isFoo = function isFoo(value: string): asserts value is 'foo' {
+>example.isFoo : Symbol(example.isFoo, Decl(expandoFunctionAsAssertion.ts, 0, 21))
+>example : Symbol(example, Decl(expandoFunctionAsAssertion.ts, 0, 0))
+>isFoo : Symbol(example.isFoo, Decl(expandoFunctionAsAssertion.ts, 0, 21))
+>isFoo : Symbol(isFoo, Decl(expandoFunctionAsAssertion.ts, 2, 15))
+>value : Symbol(value, Decl(expandoFunctionAsAssertion.ts, 2, 31))
+>value : Symbol(value, Decl(expandoFunctionAsAssertion.ts, 2, 31))
+
+  if (value !== 'foo') {
+>value : Symbol(value, Decl(expandoFunctionAsAssertion.ts, 2, 31))
+
+    throw new Error('Not foo');
+>Error : Symbol(Error, Decl(lib.es5.d.ts, --, --), Decl(lib.es5.d.ts, --, --))
+  }
+};
+
+example.isFoo('test');
+>example.isFoo : Symbol(example.isFoo, Decl(expandoFunctionAsAssertion.ts, 0, 21))
+>example : Symbol(example, Decl(expandoFunctionAsAssertion.ts, 0, 0))
+>isFoo : Symbol(example.isFoo, Decl(expandoFunctionAsAssertion.ts, 0, 21))
+
diff --git a/testdata/baselines/reference/compiler/expandoFunctionAsAssertion.types b/testdata/baselines/reference/compiler/expandoFunctionAsAssertion.types
new file mode 100644
index 0000000000..b5835b1efa
--- /dev/null
+++ b/testdata/baselines/reference/compiler/expandoFunctionAsAssertion.types
@@ -0,0 +1,34 @@
+//// [tests/cases/compiler/expandoFunctionAsAssertion.ts] ////
+
+=== expandoFunctionAsAssertion.ts ===
+function example() {}
+>example : { (): void; isFoo: (value: string) => asserts value is "foo"; }
+
+example.isFoo = function isFoo(value: string): asserts value is 'foo' {
+>example.isFoo = function isFoo(value: string): asserts value is 'foo' {  if (value !== 'foo') {    throw new Error('Not foo');  }} : (value: string) => asserts value is "foo"
+>example.isFoo : (value: string) => asserts value is "foo"
+>example : { (): void; isFoo: (value: string) => asserts value is "foo"; }
+>isFoo : (value: string) => asserts value is "foo"
+>function isFoo(value: string): asserts value is 'foo' {  if (value !== 'foo') {    throw new Error('Not foo');  }} : (value: string) => asserts value is "foo"
+>isFoo : (value: string) => asserts value is "foo"
+>value : string
+
+  if (value !== 'foo') {
+>value !== 'foo' : boolean
+>value : string
+>'foo' : "foo"
+
+    throw new Error('Not foo');
+>new Error('Not foo') : Error
+>Error : ErrorConstructor
+>'Not foo' : "Not foo"
+  }
+};
+
+example.isFoo('test');
+>example.isFoo('test') : void
+>example.isFoo : (value: string) => asserts value is "foo"
+>example : { (): void; isFoo: (value: string) => asserts value is "foo"; }
+>isFoo : (value: string) => asserts value is "foo"
+>'test' : "test"
+
diff --git a/testdata/tests/cases/compiler/expandoFunctionAsAssertion.ts b/testdata/tests/cases/compiler/expandoFunctionAsAssertion.ts
new file mode 100644
index 0000000000..7cf3c2f2fd
--- /dev/null
+++ b/testdata/tests/cases/compiler/expandoFunctionAsAssertion.ts
@@ -0,0 +1,12 @@
+// @strict: true
+// @noEmit: true
+
+function example() {}
+
+example.isFoo = function isFoo(value: string): asserts value is 'foo' {
+  if (value !== 'foo') {
+    throw new Error('Not foo');
+  }
+};
+
+example.isFoo('test');