diff --git a/tsc/internal/checker/checker.go b/tsc/internal/checker/checker.go
index 950b9a8e30fb8..8548c16afc45d 100644
--- a/tsc/internal/checker/checker.go
+++ b/tsc/internal/checker/checker.go
@@ -14278,6 +14278,7 @@ func (c *Checker) produceDeferredDiagnostics() {
 }
 
 func (c *Checker) addDiagnostic(diagnostic *ast.Diagnostic) *ast.Diagnostic {
+	debug.Assert(diagnostic.Category() != diagnostics.CategoryMessage, "Cannot report a Message-category diagnostic as an error.")
 	// Discard diagnostics created while at the maximum number of recursive TypeToString invocations.
 	if c.serializationLevel < maxSerializationLevel {
 		return c.diagnostics.Add(diagnostic)