diff --git a/common.sh b/common.sh
index 014b257307..a97c560fe8 100644
--- a/common.sh
+++ b/common.sh
@@ -8,7 +8,10 @@
 
 # The number of jobs to pass to tools that can run in parallel (such as make
 # and dpkg-buildpackage
-if [[ -z ${NUM_JOBS} ]]; then
+case "${NUM_JOBS}" in
+  *[!0-9]*) NUM_JOBS='' ;;
+esac
+if [[ -z ${NUM_JOBS} ]] || [[ ${NUM_JOBS} -eq 0 ]]; then
   NUM_JOBS=$(grep -c "^processor" /proc/cpuinfo)
 fi
 # Ensure that any sub scripts we invoke get the max proc count.