diff --git a/brmsaptool.scm b/brmsaptool.scm
index 06f8bfb..9f5038e 100644
--- a/brmsaptool.scm
+++ b/brmsaptool.scm
@@ -112,22 +112,34 @@
 ;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
 ;; Months support
 
+;; Returns true if this is a valid month representation - a list with
+;; two integer elements within the allowed range.
+(define (month-valid? m)
+  (and (list? m)
+       (car m)
+       (cdr m)
+       (cadr m)
+       (nil? (cddr m))
+       (integer? (car m))
+       (integer? (cadr m))
+       (>= (car m 1000))
+       (<= (car m 9999))
+       (>= (cadr m 1))
+       (<= (cadr m 12))))
+
 (define (string->month s)
   (list 2023 1))
 
 ;; Formats (valid) month as YYYY-MM string
 (define (month->string M)
-  (let ((y (car s))
-	(m (cadr s)))
-    (if (or (< y 1000)
-	    (> y 9999)
-	    (< m 1)
-	    (> m 12))
-	(error 'string->month "Invalid month" M)
+  (if (month-valid? M)
+      (let ((y (car s))
+	    (m (cadr s)))
 	(sprintf "~A-~A~A"
 		 y
 		 (if (< m 10) " " "")
-		 m))))
+		 m))
+      (error 'string->month "Invalid month" M)))
 
 (define (month=? m n)
   #f)