diff --git a/src/source.h b/src/source.h
index b77fa022d0..9b6e74ce16 100644
--- a/src/source.h
+++ b/src/source.h
@@ -114,9 +114,19 @@ class Source {
   /// @param content the source file content
   inline Source(const Range& rng,
                 const std::string& path,
-                FileContent* content = nullptr)
+                const FileContent* content = nullptr)
       : range(rng), file_path(path), file_content(content) {}
 
+  /// @returns a Source that points to the begin range of this Source.
+  inline Source Begin() const {
+    return Source(Range{range.begin}, file_path, file_content);
+  }
+
+  /// @returns a Source that points to the end range of this Source.
+  inline Source End() const {
+    return Source(Range{range.end}, file_path, file_content);
+  }
+
   /// range is the span of text this source refers to in #file_path
   Range range;
   /// file is the optional file path this source refers to
diff --git a/src/type_determiner.cc b/src/type_determiner.cc
index 5bce843bbb..fd334c255a 100644
--- a/src/type_determiner.cc
+++ b/src/type_determiner.cc
@@ -406,9 +406,6 @@ bool TypeDeterminer::DetermineBitcast(ast::BitcastExpression* expr) {
 }
 
 bool TypeDeterminer::DetermineCall(ast::CallExpression* call) {
-  if (!DetermineResultType(call->func())) {
-    return false;
-  }
   if (!DetermineResultType(call->params())) {
     return false;
   }
@@ -436,8 +433,8 @@ bool TypeDeterminer::DetermineCall(ast::CallExpression* call) {
 
       auto callee_func_it = symbol_to_function_.find(ident->symbol());
       if (callee_func_it == symbol_to_function_.end()) {
-        diagnostics_.add_error("unable to find called function: " + name,
-                               call->source());
+        diagnostics_.add_error(
+            "v-0006: unable to find called function: " + name, call->source());
         return false;
       }
       auto* callee_func = callee_func_it->second;
@@ -558,14 +555,16 @@ bool TypeDeterminer::DetermineIdentifier(ast::IdentifierExpression* expr) {
 
   auto iter = symbol_to_function_.find(symbol);
   if (iter != symbol_to_function_.end()) {
-    // Identifier is to a function, which has no type (currently).
-    return true;
+    diagnostics_.add_error("missing '(' for function call",
+                           expr->source().End());
+    return false;
   }
 
   std::string name = builder_->Symbols().NameFor(symbol);
   if (MatchIntrinsicType(name) != IntrinsicType::kNone) {
-    // Identifier is to an intrinsic function, which has no type (currently).
-    return true;
+    diagnostics_.add_error("missing '(' for intrinsic call",
+                           expr->source().End());
+    return false;
   }
 
   diagnostics_.add_error(
diff --git a/src/type_determiner_test.cc b/src/type_determiner_test.cc
index 3fd39515c9..801260d4f9 100644
--- a/src/type_determiner_test.cc
+++ b/src/type_determiner_test.cc
@@ -396,9 +396,8 @@ TEST_F(TypeDeterminerTest, Stmt_Call_undeclared) {
 
   EXPECT_FALSE(td()->Determine());
 
-  EXPECT_EQ(
-      td()->error(),
-      "12:34 error: v-0006: identifier must be declared before use: func");
+  EXPECT_EQ(td()->error(),
+            "12:34 error: v-0006: unable to find called function: func");
 }
 
 TEST_F(TypeDeterminerTest, Stmt_VariableDecl) {
@@ -692,6 +691,27 @@ TEST_F(TypeDeterminerTest, Expr_Call_Intrinsic) {
   EXPECT_TRUE(TypeOf(call)->Is<type::F32>());
 }
 
+TEST_F(TypeDeterminerTest, Expr_DontCall_Function) {
+  Func("func", {}, ty.void_(), {}, {});
+  auto* ident = create<ast::IdentifierExpression>(
+      Source{{Source::Location{3, 3}, Source::Location{3, 8}}},
+      Symbols().Register("func"));
+  WrapInFunction(ident);
+
+  EXPECT_FALSE(td()->Determine());
+  EXPECT_EQ(td()->error(), "3:8 error: missing '(' for function call");
+}
+
+TEST_F(TypeDeterminerTest, Expr_DontCall_Intrinsic) {
+  auto* ident = create<ast::IdentifierExpression>(
+      Source{{Source::Location{3, 3}, Source::Location{3, 8}}},
+      Symbols().Register("round"));
+  WrapInFunction(ident);
+
+  EXPECT_FALSE(td()->Determine());
+  EXPECT_EQ(td()->error(), "3:8 error: missing '(' for intrinsic call");
+}
+
 TEST_F(TypeDeterminerTest, Expr_Cast) {
   Global("name", ty.f32(), ast::StorageClass::kPrivate);