diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index e5daa0e4e2..f52d471491 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -2629,6 +2629,11 @@ pub fn (mut c Checker) expr(node_ ast.Expr) ast.Type {
 			if !node.is_type {
 				node.typ = c.expr(node.expr)
 			}
+			sym := c.table.final_sym(node.typ)
+			if sym.kind == .placeholder && sym.language != .c {
+				// Allow `sizeof(C.MYSQL_TIME)` etc
+				c.error('unknown type `${sym.name}`', node.pos)
+			}
 			// c.deprecate_old_isreftype_and_sizeof_of_a_guessed_type(node.guessed_type,
 			//	node.typ, node.pos, 'sizeof')
 			return ast.u32_type
diff --git a/vlib/v/checker/tests/sizeof.out b/vlib/v/checker/tests/sizeof.out
new file mode 100644
index 0000000000..639cc246c8
--- /dev/null
+++ b/vlib/v/checker/tests/sizeof.out
@@ -0,0 +1,4 @@
+vlib/v/checker/tests/sizeof.vv:1:13: error: unknown type `BadType`
+    1 | x := sizeof(BadType)
+      |             ~~~~~~~
+    2 | println(x)
diff --git a/vlib/v/checker/tests/sizeof.vv b/vlib/v/checker/tests/sizeof.vv
new file mode 100644
index 0000000000..47bacda256
--- /dev/null
+++ b/vlib/v/checker/tests/sizeof.vv
@@ -0,0 +1,2 @@
+x := sizeof(BadType)
+println(x)
diff --git a/vlib/v/checker/tests/unknown_sizeof_type_err_b.out b/vlib/v/checker/tests/unknown_sizeof_type_err_b.out
index f6c51c4444..7ab8a395b0 100644
--- a/vlib/v/checker/tests/unknown_sizeof_type_err_b.out
+++ b/vlib/v/checker/tests/unknown_sizeof_type_err_b.out
@@ -1,4 +1,4 @@
-vlib/v/checker/tests/unknown_sizeof_type_err_b.vv:14:34: cgen error: unknown type `Zabc`
+vlib/v/checker/tests/unknown_sizeof_type_err_b.vv:14:34: error: unknown type `Zabc`
    12 |     println('size of Abc: ${sizeof[Abc]()}')
    13 |     println('size of Xyz: ${sizeof[Xyz]()}')
    14 |     println('size of Test: ${sizeof[Zabc]()}')