diff --git a/src/modify.cpp b/src/modify.cpp
index 5c37aa5c56..219c233f2b 100644
--- a/src/modify.cpp
+++ b/src/modify.cpp
@@ -911,6 +911,7 @@ void Modify::delete_fix(const char *id)
 
 int Modify::find_fix(const char *id)
 {
+  if(id==NULL) return -1;
   int ifix;
   for (ifix = 0; ifix < nfix; ifix++)
     if (strcmp(id,fix[ifix]->id) == 0) break;
@@ -1040,6 +1041,7 @@ void Modify::delete_compute(const char *id)
 
 int Modify::find_compute(const char *id)
 {
+  if(id==NULL) return -1;
   int icompute;
   for (icompute = 0; icompute < ncompute; icompute++)
     if (strcmp(id,compute[icompute]->id) == 0) break;
diff --git a/src/variable.cpp b/src/variable.cpp
index 05c55ec7c6..63fcf1dfda 100644
--- a/src/variable.cpp
+++ b/src/variable.cpp
@@ -710,6 +710,7 @@ int Variable::next(int narg, char **arg)
 
 int Variable::find(char *name)
 {
+  if(name==NULL) return -1;
   for (int i = 0; i < nvar; i++)
     if (strcmp(name,names[i]) == 0) return i;
   return -1;