diff --git a/src/codegen/llvm.zig b/src/codegen/llvm.zig
index a24c14c9c6..300a24280a 100644
--- a/src/codegen/llvm.zig
+++ b/src/codegen/llvm.zig
@@ -8897,7 +8897,7 @@ pub const FuncGen = struct {
             return union_llvm_ty.constInt(tag_int, .False);
         }
         assert(isByRef(union_ty));
-        // The llvm type of the alloca will the the named LLVM union type, which will not
+        // The llvm type of the alloca will be the named LLVM union type, and will not
         // necessarily match the format that we need, depending on which tag is active. We
         // must construct the correct unnamed struct type here and bitcast, in order to
         // then set the fields appropriately.
@@ -8922,7 +8922,7 @@ pub const FuncGen = struct {
                 const fields: [2]*llvm.Type = .{
                     field_llvm_ty, self.context.intType(8).arrayType(padding_len),
                 };
-                break :p self.context.structType(&fields, fields.len, .False);
+                break :p self.context.structType(&fields, fields.len, .True);
             };
             if (layout.tag_size == 0) {
                 const fields: [1]*llvm.Type = .{payload};
diff --git a/test/behavior.zig b/test/behavior.zig
index 6c181d38bb..6a6844a840 100644
--- a/test/behavior.zig
+++ b/test/behavior.zig
@@ -100,6 +100,7 @@ test {
     _ = @import("behavior/bugs/12928.zig");
     _ = @import("behavior/bugs/12945.zig");
     _ = @import("behavior/bugs/12984.zig");
+    _ = @import("behavior/bugs/13128.zig");
     _ = @import("behavior/byteswap.zig");
     _ = @import("behavior/byval_arg_var.zig");
     _ = @import("behavior/call.zig");
diff --git a/test/behavior/bugs/13128.zig b/test/behavior/bugs/13128.zig
new file mode 100644
index 0000000000..057270a96f
--- /dev/null
+++ b/test/behavior/bugs/13128.zig
@@ -0,0 +1,29 @@
+const std = @import("std");
+const builtin = @import("builtin");
+const expect = std.testing.expect;
+
+const U = union(enum) {
+    x: u128,
+    y: [17]u8,
+};
+
+fn foo(val: U) !void {
+    try expect(val.x == 1);
+}
+
+test "runtime union init, most-aligned field != largest" {
+    if (builtin.zig_backend == .stage2_c) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_arm) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_aarch64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_x86_64) return error.SkipZigTest; // TODO
+    if (builtin.zig_backend == .stage2_wasm) return error.SkipZigTest; // TODO
+
+    var x: u8 = 1;
+    try foo(.{ .x = x });
+
+    const val: U = @unionInit(U, "x", x);
+    try expect(val.x == 1);
+
+    const val2: U = .{ .x = x };
+    try expect(val2.x == 1);
+}