diff --git a/src/all_types.hpp b/src/all_types.hpp
index 6f36d34bfd80a54158dcdb4f6fc73a53b3dbb329..3d732d4ace12c998eceb2a7388a8ddb8a6bfcce7 100644
--- a/src/all_types.hpp
+++ b/src/all_types.hpp
@@ -1096,6 +1096,8 @@ struct TypeTableEntryUnion {
     size_t gen_union_index;
     size_t gen_tag_index;
 
+    bool have_explicit_tag_type;
+
     uint32_t union_size_bytes;
     TypeTableEntry *most_aligned_union_member;
 
diff --git a/src/analyze.cpp b/src/analyze.cpp
index 66d29634251659a54f91b643a4bc67cf06f2c0af..74dfd003d95286204ac3ba2fe1919ebb181dd810 100644
--- a/src/analyze.cpp
+++ b/src/analyze.cpp
@@ -2558,6 +2558,8 @@ static void resolve_union_zero_bits(CodeGen *g, TypeTableEntry *union_type) {
     HashMap<BigInt, AstNode *, bigint_hash, bigint_eql> occupied_tag_values = {};
 
     AstNode *enum_type_node = decl_node->data.container_decl.init_arg_expr;
+    union_type->data.unionation.have_explicit_tag_type = decl_node->data.container_decl.auto_enum ||
+        enum_type_node != nullptr;
     bool auto_layout = (union_type->data.unionation.layout == ContainerLayoutAuto);
     bool want_safety = (field_count >= 2) && (auto_layout || enum_type_node != nullptr);
     TypeTableEntry *tag_type;
diff --git a/src/ir.cpp b/src/ir.cpp
index cdb8201b2b39eefd224ded04a6f98dd880849c9b..811744e9d0e9f2f5f6a23038a17e69c1078026ab 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -14137,6 +14137,14 @@ static IrInstruction *ir_analyze_union_tag(IrAnalyze *ira, IrInstruction *source
             buf_sprintf("expected enum or union type, found '%s'", buf_ptr(&value->value.type->name)));
         return ira->codegen->invalid_instruction;
     }
+    if (!value->value.type->data.unionation.have_explicit_tag_type && !source_instr->is_gen) {
+        ErrorMsg *msg = ir_add_error(ira, source_instr, buf_sprintf("union has no associated enum"));
+        if (value->value.type->data.unionation.decl_node != nullptr) {
+            add_error_note(ira->codegen, msg, value->value.type->data.unionation.decl_node,
+                    buf_sprintf("declared here"));
+        }
+        return ira->codegen->invalid_instruction;
+    }
 
     TypeTableEntry *tag_type = value->value.type->data.unionation.tag_type;
     assert(tag_type->id == TypeTableEntryIdEnum);
diff --git a/test/compile_errors.zig b/test/compile_errors.zig
index 1bca093e791fbaa60af47ec460fe3ad7db857a6e..564ad5d5214943c2e832c9f9cd36d44b90d09a92 100644
--- a/test/compile_errors.zig
+++ b/test/compile_errors.zig
@@ -1,6 +1,19 @@
 const tests = @import("tests.zig");
 
 pub fn addCases(cases: &tests.CompileErrorContext) void {
+    cases.add("@tagName used on union with no associated enum tag",
+        \\const FloatInt = extern union {
+        \\    Float: f32,
+        \\    Int: i32,
+        \\};
+        \\export fn entry() void {
+        \\    var fi = FloatInt{.Float = 123.45};
+        \\    var tagName = @tagName(fi);
+        \\}
+    ,
+        ".tmp_source.zig:7:19: error: union has no associated enum",
+        ".tmp_source.zig:1:18: note: declared here");
+
     cases.add("returning error from void async function",
         \\const std = @import("std");
         \\export fn entry() void {