diff --git a/config.lib b/config.lib
--- a/config.lib
+++ b/config.lib
@@ -1404,6 +1404,9 @@ make_compiler_cflags() {
 			# warning: narrowing conversion of '...' from 'unsigned int' to 'int' inside { } [-Wnarrowing]
 			# They are valid according to the C++ standard, but useless.
 			cxxflags="$cxxflags -Wno-narrowing"
+		fi
+
+		if [ $cc_version -ge 407 ]; then
 			# Disable bogus 'attempt to free a non-heap object' warning
 			flags="$flags -Wno-free-nonheap-object"
 		fi
diff --git a/src/saveload/saveload.cpp b/src/saveload/saveload.cpp
--- a/src/saveload/saveload.cpp
+++ b/src/saveload/saveload.cpp
@@ -1484,6 +1484,8 @@ size_t SlCalcObjMemberLength(const void 
 	return 0;
 }
 
+#ifdef OTTD_ASSERT
+
 /**
  * Check whether the variable size of the variable in the saveload configuration
  * matches with the actual variable size.
@@ -1524,9 +1526,13 @@ static bool IsVariableSizeRight(const Sa
 	}
 }
 
+#endif /* OTTD_ASSERT */
+
 bool SlObjectMember(void *ptr, const SaveLoad *sld)
 {
+#ifdef OTTD_ASSERT
 	assert(IsVariableSizeRight(sld));
+#endif
 
 	VarType conv = GB(sld->conv, 0, 8);
 	switch (sld->cmd) {