diff --git a/include/rtl/string.hxx b/include/rtl/string.hxx
index e5ac7d6821d7..8b7eca4b34dd 100644
--- a/include/rtl/string.hxx
+++ b/include/rtl/string.hxx
@@ -362,7 +362,8 @@ public:
       @since LibreOffice 3.5
     */
     sal_Char operator [](sal_Int32 index) const {
-        assert(index >= 0 && index < getLength());
+        // silence spurious -Werror=strict-overflow warnings from GCC 4.8.2
+        assert(index >= 0 && static_cast<sal_uInt32>(index) < static_cast<sal_uInt32>(getLength()));
         return getStr()[index];
     }
 
diff --git a/include/rtl/ustring.hxx b/include/rtl/ustring.hxx
index 327f455f1648..79ddfb0ca788 100644
--- a/include/rtl/ustring.hxx
+++ b/include/rtl/ustring.hxx
@@ -417,7 +417,8 @@ public:
       @since LibreOffice 3.5
     */
     sal_Unicode operator [](sal_Int32 index) const {
-        assert(index >= 0 && index < getLength());
+        // silence spurious -Werror=strict-overflow warnings from GCC 4.8.2
+        assert(index >= 0 && static_cast<sal_uInt32>(index) < static_cast<sal_uInt32>(getLength()));
         return getStr()[index];
     }