diff --git a/src/randr.c b/src/randr.c
index 6b6cd67d..dd30925b 100644
--- a/src/randr.c
+++ b/src/randr.c
@@ -447,10 +447,12 @@ void init_ws_for_output(Output *output, Con *content) {
         if (!exists) {
             /* Set ->num to the number of the workspace, if the name actually
              * is a number or starts with a number */
-            long parsed_num = strtol(ws->name, NULL, 10);
+            char *endptr = NULL;
+            long parsed_num = strtol(ws->name, &endptr, 10);
             if (parsed_num == LONG_MIN ||
                 parsed_num == LONG_MAX ||
-                parsed_num <= 0)
+                parsed_num < 0 ||
+                endptr == ws->name)
                 ws->num = -1;
             else ws->num = parsed_num;
             LOG("Used number %d for workspace with name %s\n", ws->num, ws->name);
diff --git a/src/workspace.c b/src/workspace.c
index 963fa64b..27899a37 100644
--- a/src/workspace.c
+++ b/src/workspace.c
@@ -49,11 +49,12 @@ Con *workspace_get(const char *num, bool *created) {
         workspace->name = sstrdup(num);
         /* We set ->num to the number if this workspace’s name consists only of
          * a positive number. Otherwise it’s a named ws and num will be -1. */
-
-        long parsed_num = strtol(num, NULL, 10);
+        char *endptr = NULL;
+        long parsed_num = strtol(num, &endptr, 10);
         if (parsed_num == LONG_MIN ||
             parsed_num == LONG_MAX ||
-            parsed_num < 0)
+            parsed_num < 0 ||
+            endptr == num)
             workspace->num = -1;
         else workspace->num = parsed_num;
         LOG("num = %d\n", workspace->num);
diff --git a/testcases/t/17-workspace.t b/testcases/t/17-workspace.t
index 19e2df34..3c3b6cc6 100644
--- a/testcases/t/17-workspace.t
+++ b/testcases/t/17-workspace.t
@@ -98,4 +98,23 @@ cmd 'workspace "prev"';
 ok(workspace_exists('prev'), 'workspace "prev" exists');
 is(focused_ws(), 'prev', 'now on workspace prev');
 
+#####################################################################
+# check that the numbers are assigned/recognized correctly
+#####################################################################
+
+cmd "workspace 3: $tmp";
+my $ws = get_ws("3: $tmp");
+ok(defined($ws), "workspace 3: $tmp was created");
+is($ws->{num}, 3, 'workspace number is 3');
+
+cmd "workspace 0: $tmp";
+my $ws = get_ws("0: $tmp");
+ok(defined($ws), "workspace 0: $tmp was created");
+is($ws->{num}, 0, 'workspace number is 0');
+
+cmd "workspace aa: $tmp";
+my $ws = get_ws("aa: $tmp");
+ok(defined($ws), "workspace aa: $tmp was created");
+is($ws->{num}, -1, 'workspace number is -1');
+
 done_testing;