diff --git a/src/force.cpp b/src/force.cpp
index cef61fc557..44c369acca 100644
--- a/src/force.cpp
+++ b/src/force.cpp
@@ -747,7 +747,14 @@ void Force::boundsbig(char *str, bigint nmax, bigint &nlo, bigint &nhi,
 
 double Force::numeric(const char *file, int line, char *str)
 {
+  if (!str)
+    error->all(file,line,"Expected floating point parameter "
+               "in input script or data file");
   int n = strlen(str);
+  if (n == 0)
+    error->all(file,line,"Expected floating point parameter "
+               "in input script or data file");
+
   for (int i = 0; i < n; i++) {
     if (isdigit(str[i])) continue;
     if (str[i] == '-' || str[i] == '+' || str[i] == '.') continue;
@@ -767,7 +774,14 @@ double Force::numeric(const char *file, int line, char *str)
 
 int Force::inumeric(const char *file, int line, char *str)
 {
+  if (!str) 
+    error->all(file,line,
+               "Expected integer parameter in input script or data file");
   int n = strlen(str);
+  if (n == 0) 
+    error->all(file,line,
+               "Expected integer parameter in input script or data file");
+
   for (int i = 0; i < n; i++) {
     if (isdigit(str[i]) || str[i] == '-' || str[i] == '+') continue;
     error->all(file,line,
@@ -785,14 +799,46 @@ int Force::inumeric(const char *file, int line, char *str)
 
 bigint Force::bnumeric(const char *file, int line, char *str)
 {
+  if (!str) 
+    error->all(file,line,
+               "Expected integer parameter in input script or data file");
   int n = strlen(str);
+  if (n == 0) 
+    error->all(file,line,
+               "Expected integer parameter in input script or data file");
+
   for (int i = 0; i < n; i++) {
     if (isdigit(str[i]) || str[i] == '-' || str[i] == '+') continue;
     error->all(file,line,
                "Expected integer parameter in input script or data file");
   }
 
-  return ATOLL(str);
+  return ATOBIGINT(str);
+}
+
+/* ----------------------------------------------------------------------
+   read a tag integer value from a string
+   generate an error if not a legitimate integer value
+   called by various commands to check validity of their arguments
+------------------------------------------------------------------------- */
+
+tagint Force::tnumeric(const char *file, int line, char *str)
+{
+  if (!str) 
+    error->all(file,line,
+               "Expected integer parameter in input script or data file");
+  int n = strlen(str);
+  if (n == 0) 
+    error->all(file,line,
+               "Expected integer parameter in input script or data file");
+
+  for (int i = 0; i < n; i++) {
+    if (isdigit(str[i]) || str[i] == '-' || str[i] == '+') continue;
+    error->all(file,line,
+               "Expected integer parameter in input script or data file");
+  }
+
+  return ATOTAGINT(str);
 }
 
 /* ----------------------------------------------------------------------
diff --git a/src/force.h b/src/force.h
index a9efb08fbf..bf364f253b 100644
--- a/src/force.h
+++ b/src/force.h
@@ -105,6 +105,7 @@ class Force : protected Pointers {
   double numeric(const char *, int, char *);
   int inumeric(const char *, int, char *);
   bigint bnumeric(const char *, int, char *);
+  tagint tnumeric(const char *, int, char *);
 
   FILE *open_potential(const char *);
   const char *potname(const char *);
diff --git a/src/group.cpp b/src/group.cpp
index 4144108f24..3b494a0b42 100644
--- a/src/group.cpp
+++ b/src/group.cpp
@@ -198,18 +198,12 @@ void Group::assign(int narg, char **arg)
       else error->all(FLERR,"Illegal group command");
       
       tagint bound1,bound2;
-      if (sizeof(tagint) == sizeof(smallint))
-        bound1 = force->inumeric(FLERR,arg[3]);
-      else
-        bound1 = force->bnumeric(FLERR,arg[3]);
+      bound1 = force->tnumeric(FLERR,arg[3]);
       bound2 = -1;
 
       if (condition == BETWEEN) {
         if (narg != 5) error->all(FLERR,"Illegal group command");
-        if (sizeof(tagint) == sizeof(smallint))
-          bound2 = force->inumeric(FLERR,arg[4]);
-        else
-          bound2 = force->bnumeric(FLERR,arg[4]);
+        bound2 = force->tnumeric(FLERR,arg[4]);
       } else if (narg != 4) error->all(FLERR,"Illegal group command");
 
       int *attribute = NULL;
@@ -284,13 +278,15 @@ void Group::assign(int narg, char **arg)
 
       for (int iarg = 2; iarg < narg; iarg++) {
         if (strchr(arg[iarg],':')) {
-          start = ATOTAGINT(strtok(arg[iarg],":")); 
-          stop = ATOTAGINT(strtok(NULL,":"));
+          ptr = strtok(arg[iarg],":"); 
+          start = force->tnumeric(FLERR,ptr); 
+          ptr = strtok(NULL,":"); 
+          stop = force->tnumeric(FLERR,ptr); 
           ptr = strtok(NULL,":");
-          if (ptr) delta = ATOTAGINT(ptr);
+          if (ptr) delta = force->tnumeric(FLERR,ptr);
           else delta = 1;
         } else {
-          start = stop = ATOTAGINT(arg[iarg]);
+          start = stop = force->tnumeric(FLERR,arg[iarg]);
           delta = 1;
         }