--- a/configure.in
+++ b/configure.in
@@ -1285,21 +1285,23 @@ if test "$GNU_CXX"; then
     # Turn on GNU-specific warnings:
     # -Wall - turn on a lot of warnings
     # -pedantic - this is turned on below
     # -Wpointer-arith - enabled with -pedantic, but good to have even if not
     # -Woverloaded-virtual - ???
     # -Werror=return-type - catches missing returns, zero false positives
     # -Wtype-limits - catches overflow bugs, few false positives
     # -Wempty-body - catches bugs, e.g. "if (c); foo();", few false positives
+    # -Werror=conversion-null - catches conversions between NULL and non-pointer types
     #
     _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wall -Wpointer-arith -Woverloaded-virtual"
     MOZ_CXX_SUPPORTS_WARNING(-W, error=return-type, ac_cxx_has_werror_return_type)
     MOZ_CXX_SUPPORTS_WARNING(-W, type-limits, ac_cxx_has_wtype_limits)
     MOZ_CXX_SUPPORTS_WARNING(-W, empty-body, ac_cxx_has_wempty_body)
+    MOZ_CXX_SUPPORTS_WARNING(-W, error=conversion-null, ac_cxx_has_werror_conversion_null)
 
     # Turn off the following warnings that -Wall/-pedantic turn on:
     # -Wno-ctor-dtor-privacy - ???
     # -Wno-overlength-strings - we exceed the minimum maximum length frequently 
     # -Wno-invalid-offsetof - we use offsetof on non-POD types frequently
     # -Wno-variadic-macros - ???
     #
     _WARNINGS_CXXFLAGS="${_WARNINGS_CXXFLAGS} -Wno-ctor-dtor-privacy"