diff --git a/js/src/configure.in b/js/src/configure.in
index ab2ac43d789..7c72702ba9b 100644
--- a/js/src/configure.in
+++ b/js/src/configure.in
@@ -71,27 +71,36 @@ dnl ========================================================
 USE_PTHREADS=
 _PTHREAD_LDFLAGS=""
 
-dnl Do not allow a separate objdir build if a srcdir build exists.
+dnl Do not allow objdir == srcdir builds
 dnl ==============================================================
 _topsrcdir=`cd $srcdir; pwd`
 _objdir=`pwd`
 
-if test "$_topsrcdir" != "$_objdir"
+if test "$_topsrcdir" = "$_objdir"
 then
-  # Check for a couple representative files in the source tree
-  _conflict_files=
-  for file in $_topsrcdir/Makefile $_topsrcdir/config/autoconf.mk; do
-    if test -f $file; then
-      _conflict_files="$_conflict_files $file"
-    fi
+  echo "  ***"
+  echo "  * Building directly in the main source directory is not allowed."
+  echo "  *"
+  echo "  * To build, you must run configure from a separate directory"
+  echo "  * (referred to as an object directory)."
+  echo "  ***"
+  exit 1
+fi
+
+# Check for a couple representative files in the source tree
+_conflict_files=
+for file in $_topsrcdir/Makefile $_topsrcdir/config/autoconf.mk; do
+  if test -f $file; then
+    _conflict_files="$_conflict_files $file"
+  fi
+done
+if test "$_conflict_files"; then
+  echo "***"
+  echo "*   Your source tree contains these files:"
+  for file in $_conflict_files; do
+    echo "*         $file"
   done
-  if test "$_conflict_files"; then
-    echo "***"
-    echo "*   Your source tree contains these files:"
-    for file in $_conflict_files; do
-      echo "*         $file"
-    done
-    cat 1>&2 <<-EOF
+  cat 1>&2 <<-EOF
 	*   This indicates that you previously built in the source tree.
 	*   A source tree build can confuse the separate objdir build.
 	*
@@ -100,9 +109,8 @@ then
 	*     2. gmake distclean
 	***
 	EOF
-    exit 1
-    break
-  fi
+  exit 1
+  break
 fi
 MOZ_BUILD_ROOT=`pwd`