diff --git a/hash.c b/hash.c
index 3ef6668f5f..c4a2c0f634 100644
--- a/hash.c
+++ b/hash.c
@@ -3034,6 +3034,7 @@ ruby_unsetenv(const char *name)
  *
  * Sets the environment variable +name+ to +value+.  If the value given is
  * +nil+ the environment variable is deleted.
+ * +name+ must be a string.
  *
  */
 static VALUE
diff --git a/process.c b/process.c
index 88606c34eb..c52e6ac896 100644
--- a/process.c
+++ b/process.c
@@ -3950,6 +3950,8 @@ rb_f_system(int argc, VALUE *argv)
  *    env: hash
  *      name => val : set the environment variable
  *      name => nil : unset the environment variable
+ *
+ *      the keys must be strings.
  *    command...:
  *      commandline                 : command line string which is passed to the standard shell
  *      cmdname, arg1, ...          : command name and one or more arguments (This form does not use the shell. See below for caveats.)