diff --git a/src/core/transaction.c b/src/core/transaction.c
index 1f420fe176..3cfa974fda 100644
--- a/src/core/transaction.c
+++ b/src/core/transaction.c
@@ -341,6 +341,9 @@ static char* merge_unit_ids(const char* unit_log_field, char * const* pairs) {
                 size += next + 1;
         }
 
+        if (!ans)
+                return strdup("");
+
         return TAKE_PTR(ans);
 }