diff --git a/vlib/v/ast/ast.v b/vlib/v/ast/ast.v
index 0b4c7d5f37..53e34bd0c2 100644
--- a/vlib/v/ast/ast.v
+++ b/vlib/v/ast/ast.v
@@ -1022,11 +1022,11 @@ pub mut:
 
 pub struct SizeOf {
 pub:
-	is_type   bool
-	typ       table.Type
-	type_name string
-	expr      Expr
-	pos       token.Position
+	is_type bool
+	expr    Expr // checker uses this to set typ
+	pos     token.Position
+pub mut:
+	typ table.Type
 }
 
 pub struct Likely {
diff --git a/vlib/v/checker/checker.v b/vlib/v/checker/checker.v
index e23c72f563..7fd8130cb2 100644
--- a/vlib/v/checker/checker.v
+++ b/vlib/v/checker/checker.v
@@ -3357,6 +3357,9 @@ pub fn (mut c Checker) expr(node ast.Expr) table.Type {
 			return c.selector_expr(mut node)
 		}
 		ast.SizeOf {
+			if !node.is_type {
+				node.typ = c.expr(node.expr)
+			}
 			return table.u32_type
 		}
 		ast.SqlExpr {
diff --git a/vlib/v/fmt/fmt.v b/vlib/v/fmt/fmt.v
index 49e5d094f2..eb3c5a1b82 100644
--- a/vlib/v/fmt/fmt.v
+++ b/vlib/v/fmt/fmt.v
@@ -1122,23 +1122,22 @@ pub fn (mut f Fmt) expr(node ast.Expr) {
 			f.write(node.field_name)
 		}
 		ast.SizeOf {
+			f.write('sizeof(')
 			if node.is_type {
-				f.write('sizeof(')
-				if node.type_name != '' {
-					if f.is_external_name(node.type_name) {
-						f.write(node.type_name)
+				sym := f.table.get_type_symbol(node.typ)
+				if sym.name != '' {
+					if f.is_external_name(sym.name) {
+						f.write(sym.name)
 					} else {
-						f.write(f.short_module(node.type_name))
+						f.write(f.short_module(sym.name))
 					}
 				} else {
 					f.write(f.table.type_to_str(node.typ))
 				}
-				f.write(')')
 			} else {
-				f.write('sizeof(')
 				f.expr(node.expr)
-				f.write(')')
 			}
+			f.write(')')
 		}
 		ast.SqlExpr {
 			// sql app.db { select from Contributor where repo == id && user == 0 }
diff --git a/vlib/v/gen/cgen.v b/vlib/v/gen/cgen.v
index b0f345062e..58034b8c85 100644
--- a/vlib/v/gen/cgen.v
+++ b/vlib/v/gen/cgen.v
@@ -2719,29 +2719,9 @@ fn (mut g Gen) expr(node ast.Expr) {
 			g.select_expr(node)
 		}
 		ast.SizeOf {
-			if node.is_type {
-				node_typ := g.unwrap_generic(node.typ)
-				mut styp := node.type_name
-				if styp.starts_with('C.') {
-					styp = styp[2..]
-				}
-				if node.type_name == '' || node.typ.has_flag(.generic) {
-					styp = g.typ(node_typ)
-				} else {
-					sym := g.table.get_type_symbol(node_typ)
-					if sym.kind == .struct_ {
-						info := sym.info as table.Struct
-						if !info.is_typedef {
-							styp = 'struct ' + styp
-						}
-					}
-				}
-				g.write('/*SizeOfType*/ sizeof(${util.no_dots(styp)})')
-			} else {
-				g.write('/*SizeOfVar*/ sizeof(')
-				g.expr(node.expr)
-				g.write(')')
-			}
+			node_typ := g.unwrap_generic(node.typ)
+			styp := g.typ(node_typ)
+			g.write('/*SizeOf*/ sizeof(${util.no_dots(styp)})')
 		}
 		ast.SqlExpr {
 			g.sql_select_expr(node)
diff --git a/vlib/v/parser/pratt.v b/vlib/v/parser/pratt.v
index 56873f8901..02069735e9 100644
--- a/vlib/v/parser/pratt.v
+++ b/vlib/v/parser/pratt.v
@@ -158,10 +158,9 @@ pub fn (mut p Parser) expr(precedence int) ast.Expr {
 			pos := p.tok.position()
 			p.next() // sizeof
 			p.check(.lpar)
-			if !p.tok.can_start_type(table.builtin_type_names) {
-				if p.tok.kind == .name {
-					p.mark_var_as_used(p.tok.lit)
-				}
+			is_known_var := p.mark_var_as_used(p.tok.lit)
+			// assume mod. prefix leads to a type
+			if is_known_var || !(p.known_import(p.tok.lit) || p.tok.kind.is_start_of_type()) {
 				expr := p.expr(0)
 				node = ast.SizeOf{
 					is_type: false
@@ -169,6 +168,7 @@ pub fn (mut p Parser) expr(precedence int) ast.Expr {
 					pos: pos
 				}
 			} else {
+				p.register_used_import(p.tok.lit)
 				save_expr_mod := p.expr_mod
 				p.expr_mod = ''
 				sizeof_type := p.parse_type()
@@ -176,7 +176,6 @@ pub fn (mut p Parser) expr(precedence int) ast.Expr {
 				node = ast.SizeOf{
 					is_type: true
 					typ: sizeof_type
-					type_name: p.table.get_type_symbol(sizeof_type).name
 					pos: pos
 				}
 			}
diff --git a/vlib/v/tests/sizeof_test.v b/vlib/v/tests/sizeof_test.v
index 372f13250b..df17734036 100644
--- a/vlib/v/tests/sizeof_test.v
+++ b/vlib/v/tests/sizeof_test.v
@@ -1,7 +1,11 @@
 import math
+import flag
 
 struct S1 {
-	i voidptr
+	p voidptr
+}
+struct S2 {
+	i int
 }
 
 fn test_math_sizeof() {
@@ -10,8 +14,12 @@ fn test_math_sizeof() {
 }
 
 fn test_sizeof() {
-	// depends on compiler
-	assert sizeof(`€`) in [u32(2), 4]
+	assert sizeof(rune) == 4
+	assert sizeof([44]byte) == 44
+	assert sizeof(`€`) == 4
 	// depends on -m32/64
 	assert sizeof(S1) in [u32(4), 8]
+	s := S2{}
+	assert sizeof(s.i) == 4
+	assert sizeof(flag.Flag) > 4
 }