diff --git a/fuzzers/tint_ast_fuzzer/mutation.cc b/fuzzers/tint_ast_fuzzer/mutation.cc
index 0e5247100f..59c8878c83 100644
--- a/fuzzers/tint_ast_fuzzer/mutation.cc
+++ b/fuzzers/tint_ast_fuzzer/mutation.cc
@@ -32,7 +32,9 @@ std::unique_ptr<Mutation> Mutation::FromMessage(
           message.replace_identifier());
     case protobufs::Mutation::MUTATION_NOT_SET:
       assert(false && "Mutation is not set");
+      break;
   }
+  return nullptr;
 }
 
 }  // namespace ast_fuzzer