diff --git a/src/sleep.c b/src/sleep.c
index 039a4b8..b560a59 100644
--- a/src/sleep.c
+++ b/src/sleep.c
@@ -2,7 +2,7 @@
 #include <stdio.h>
 #include <errno.h>
 #include <time.h>
-
+#include <ctype.h>
 
 #include "support.h"
 
@@ -18,15 +18,22 @@ int main(int argc, char **argv)
 	throw(NEEDARG_FAIL, mastrcat(argv[0], " int"));
 	}
     
+    /* have we received a positive integer? */
+    /* subtract one from strlen result to account for zero indexing */
+    for(i = (strlen(argv[1])-1); i >= 0; i--)
+	{
+	if(!isdigit(argv[1][i]))
+	    {
+	    throw(NEEDARG_FAIL, mastrcat(argv[0], " int"));
+	    }
+	}
+    
+    
     i = (int) (strtol(argv[1], (char **) NULL, 10));
     if(errno == ERANGE)
 	{
 	throw(TIME_FAIL, mastrcat(argv[0], " ERANGE error."));
 	}
-    if(i < 0)
-	    {
-	    throw(MISC_FAIL, mastrcat(argv[0], ": need positive number."));
-	    }
 
 
     sleep(i);