diff --git a/src/region.cpp b/src/region.cpp
index b533d45ea7..b61a5f8905 100644
--- a/src/region.cpp
+++ b/src/region.cpp
@@ -302,7 +302,7 @@ void Region::rotate(double &x, double &y, double &z, double angle)
 
 void Region::options(int narg, char **arg)
 {
-  if (narg < 0) error->all(FLERR, "Illegal region command");
+  if (narg < 0) utils::missing_cmd_args(FLERR, "region", error);
 
   // option defaults
 
@@ -316,46 +316,46 @@ void Region::options(int narg, char **arg)
   int iarg = 0;
   while (iarg < narg) {
     if (strcmp(arg[iarg], "units") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal region command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "region units", error);
       if (strcmp(arg[iarg + 1], "box") == 0)
         scaleflag = 0;
       else if (strcmp(arg[iarg + 1], "lattice") == 0)
         scaleflag = 1;
       else
-        error->all(FLERR, "Illegal region command");
+        error->all(FLERR, "Illegal region units: {}", arg[iarg+1]);
       iarg += 2;
     } else if (strcmp(arg[iarg], "side") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal region command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "region side", error); 
       if (strcmp(arg[iarg + 1], "in") == 0)
         interior = 1;
       else if (strcmp(arg[iarg + 1], "out") == 0)
         interior = 0;
       else
-        error->all(FLERR, "Illegal region command");
+        error->all(FLERR, "Illegal region side: {}", arg[iarg+1]);
       iarg += 2;
 
     } else if (strcmp(arg[iarg], "move") == 0) {
-      if (iarg + 4 > narg) error->all(FLERR, "Illegal region command");
+      if (iarg + 4 > narg) utils::missing_cmd_args(FLERR, "region move", error);
       if (strcmp(arg[iarg + 1], "NULL") != 0) {
         if (strstr(arg[iarg + 1], "v_") != arg[iarg + 1])
-          error->all(FLERR, "Illegal region command");
+          error->all(FLERR, "Illegal region move x displacement variable: {}", arg[iarg+1]);
         xstr = utils::strdup(&arg[iarg + 1][2]);
       }
       if (strcmp(arg[iarg + 2], "NULL") != 0) {
         if (strstr(arg[iarg + 2], "v_") != arg[iarg + 2])
-          error->all(FLERR, "Illegal region command");
+          error->all(FLERR, "Illegal region move y displacement variable: {}", arg[iarg+2]);
         ystr = utils::strdup(&arg[iarg + 2][2]);
       }
       if (strcmp(arg[iarg + 3], "NULL") != 0) {
         if (strstr(arg[iarg + 3], "v_") != arg[iarg + 3])
-          error->all(FLERR, "Illegal region command");
+          error->all(FLERR, "Illegal region move z displacement variable: {}", arg[iarg+3]);
         zstr = utils::strdup(&arg[iarg + 3][2]);
       }
       moveflag = 1;
       iarg += 4;
 
     } else if (strcmp(arg[iarg], "rotate") == 0) {
-      if (iarg + 8 > narg) error->all(FLERR, "Illegal region command");
+      if (iarg + 8 > narg) utils::missing_cmd_args(FLERR, "region rotate", error);
       if (strstr(arg[iarg + 1], "v_") != arg[iarg + 1]) error->all(FLERR, "Illegal region command");
       tstr = utils::strdup(&arg[iarg + 1][2]);
       point[0] = utils::numeric(FLERR, arg[iarg + 2], false, lmp);
@@ -368,15 +368,14 @@ void Region::options(int narg, char **arg)
       iarg += 8;
 
     } else if (strcmp(arg[iarg], "open") == 0) {
-      if (iarg + 2 > narg) error->all(FLERR, "Illegal region command");
+      if (iarg + 2 > narg) utils::missing_cmd_args(FLERR, "region open", error);
       int iface = utils::inumeric(FLERR, arg[iarg + 1], false, lmp);
-      if (iface < 1 || iface > 6) error->all(FLERR, "Illegal region command");
+      if (iface < 1 || iface > 6) error->all(FLERR, "Illegal region open face index: {}", iface);
       // additional checks on valid face index are done by region classes
       open_faces[iface - 1] = 1;
       openflag = 1;
       iarg += 2;
-    } else
-      error->all(FLERR, "Illegal region command");
+    } else error->all(FLERR, "Illegal region command argument: {}", arg[iarg]);
   }
 
   // error check
diff --git a/unittest/commands/test_regions.cpp b/unittest/commands/test_regions.cpp
index a021c6a338..36d251e542 100644
--- a/unittest/commands/test_regions.cpp
+++ b/unittest/commands/test_regions.cpp
@@ -203,7 +203,8 @@ TEST_F(RegionTest, DeathTests)
     TEST_FAILURE(".*ERROR: Illegal region cylinder axis: xx.*", command("region reg9 cylinder  xx 0 0 1 0 1 open 1 units box"););
 
     TEST_FAILURE(".*ERROR: Unrecognized region style 'xxx'.*", command("region new1 xxx"););
-    TEST_FAILURE(".*ERROR: Illegal region command.*", command("region new1 block 0 1"););
+    //TEST_FAILURE(".*ERROR: Illegal region command.*", command("region new1 block 0 1"););
+    TEST_FAILURE(".*ERROR: Illegal region command: missing argument\\(s\\).*", command("region new1 block 0 1"););
     TEST_FAILURE(".*ERROR: Delete region new3 does not exist.*", command("region new3 delete"););
 }