diff --git a/tests/base64/base64.cpp b/tests/base64/base64.cpp
index d75be1108a..9405406887 100644
--- a/tests/base64/base64.cpp
+++ b/tests/base64/base64.cpp
@@ -291,16 +291,17 @@ void Base64TestCase::DecodeInvalid()
     CPPUNIT_ASSERT_EQUAL( wxCONV_FAILED, rc);
     CPPUNIT_ASSERT_EQUAL( 0, posErr );
 
-    posErr = (size_t)-1;
+    const size_t POS_INVALID = -1;
+    posErr = POS_INVALID;
     rc = wxBase64Decode(NULL, 0, " QQ==", wxNO_LEN,
                         wxBase64DecodeMode_SkipWS, &posErr);
     CPPUNIT_ASSERT_EQUAL( 1, rc );
-    CPPUNIT_ASSERT_EQUAL( -1, posErr );
+    CPPUNIT_ASSERT_EQUAL( POS_INVALID, posErr );
 
     rc = wxBase64Decode(NULL, 0, "? QQ==", wxNO_LEN,
                         wxBase64DecodeMode_Relaxed, &posErr);
     CPPUNIT_ASSERT_EQUAL( 1, rc );
-    CPPUNIT_ASSERT_EQUAL( -1, posErr );
+    CPPUNIT_ASSERT_EQUAL( POS_INVALID, posErr );
 
     CPPUNIT_ASSERT( !wxBase64Decode("wxGetApp()").GetDataLen() );
 }