diff --git a/vlib/v/gen/c/str_intp.v b/vlib/v/gen/c/str_intp.v
index a3daef03bb..bcc3bf9106 100644
--- a/vlib/v/gen/c/str_intp.v
+++ b/vlib/v/gen/c/str_intp.v
@@ -91,6 +91,8 @@ fn (mut g Gen) str_format(node ast.StringInterLiteral, i int) (u64, string) {
 				ast.i64_type { fmt_type = .si_i64 }
 				ast.u64_type { fmt_type = .si_u64 }
 				ast.u32_type { fmt_type = .si_u32 }
+				ast.usize_type { fmt_type = .si_u64 }
+				ast.isize_type { fmt_type = .si_i64 }
 				else { fmt_type = .si_i32 }
 			}
 		}
diff --git a/vlib/v/tests/integer_size_test.v b/vlib/v/tests/integer_size_test.v
index 43b1a9c500..6794e11cbc 100644
--- a/vlib/v/tests/integer_size_test.v
+++ b/vlib/v/tests/integer_size_test.v
@@ -9,6 +9,9 @@ fn test_usize() {
 	u++
 	assert u == 5
 	assert u.str() == '5'
+	$if x64 {
+		assert '${usize(0x140000000):X}' == '140000000'
+	}
 }
 
 fn test_isize() {
@@ -19,4 +22,7 @@ fn test_isize() {
 	i += 2
 	assert i == -3
 	assert i.str() == '-3'
+	$if x64 {
+		assert '${isize(0x140000000):X}' == '140000000'
+	}
 }