--- nsprpub/configure.in
+++ nsprpub/configure.in
@@ -57,7 +57,7 @@
 USE_USER_PTHREADS=
 USE_NSPR_THREADS=
 USE_N32=
-USE_64=
+USE_64=maybe
 USE_CPLUS=
 USE_IPV6=
 USE_MDUPDATE=
@@ -222,11 +222,26 @@
     fi ])
 
 AC_ARG_ENABLE(64bit,
-    [  --enable-64bit          Enable 64-bit support (on certain platforms)],
-    [ if test "$enableval" = "yes"; then
-	    USE_64=1
+    [  --disable-64bit         Disable 64-bit support (on 64-bit platforms)],
+    [ if test "$enableval" = "no"; then
+	    USE_64=
+      else
+	    USE_64=1
       fi ])
 
+if test "${USE_64}"; then
+    AC_MSG_CHECKING(for 64-bit OS)
+    AC_TRY_COMPILE([],[int assert[(sizeof(long) == 8) ? 1: -1]],
+        result="yes", result="no")
+    AC_MSG_RESULT("$result")
+    if test "$result" = "no" && test "$USE_64" = 1; then
+        AC_MSG_ERROR([Can't --enable-64bit on non 64-bit platforms])
+    fi
+    if test "$result" = "yes"; then
+        USE_64=1
+    fi
+fi
+
 AC_ARG_ENABLE(mdupdate,
     [  --enable-mdupdate       Enable use of certain compilers' mdupdate feature],
     [ if test "$enableval" = "yes"; then