diff --git a/sugar/file.rkt b/sugar/file.rkt
index dfcd6a6..7be5380 100644
--- a/sugar/file.rkt
+++ b/sugar/file.rkt
@@ -29,6 +29,10 @@
 ;; does path have a certain extension, case-insensitively
 (define+provide+safe (has-ext? x ext)
   (pathish? stringish? . -> . boolean?)
+  (unless (pathish? x)
+    (raise-argument-error 'has-ext? "pathish?" x))
+  (unless (stringish? ext)
+    (raise-argument-error 'has-ext? "stringish?" ext))
   (define ext-of-path (filename-extension (->path x)))
   (and ext-of-path (equal? (string-downcase (bytes->string/utf-8 ext-of-path)) (string-downcase (->string ext)))))
 
@@ -37,6 +41,8 @@
 ;; (consistent with filename-extension behavior)
 (define+provide+safe (get-ext x)
   (pathish? . -> . (or/c #f string?))
+  (unless (pathish? x)
+    (raise-argument-error 'get-ext "pathish?" x))
   (let ([fe-result (filename-extension (->path x))])
     (and fe-result (bytes->string/utf-8 fe-result))))
 
@@ -49,6 +55,8 @@
 
 (define+provide+safe (has-binary-ext? x)
   (pathish? . -> . boolean?)
+  (unless (pathish? x)
+    (raise-argument-error 'has-binary-ext? "pathish?" x))
   (let ([x (->path x)])
     (and (ormap (λ(ext) (has-ext? x ext)) binary-extensions) #t)))
 
@@ -57,6 +65,10 @@
 ;; use local contract here because this function is used within module
 (define+provide+safe (add-ext x ext)
   (stringish? stringish? . -> . pathish?)
+  (unless (stringish? x)
+    (raise-argument-error 'add-ext "stringish?" x))
+  (unless (stringish? ext)
+    (raise-argument-error 'add-ext "stringish?" ext))
   (->path (string-append (->string x) "." (->string ext))))
 
 
diff --git a/sugar/test/main.rkt b/sugar/test/main.rkt
index d9e735d..0af68ae 100644
--- a/sugar/test/main.rkt
+++ b/sugar/test/main.rkt
@@ -144,12 +144,18 @@
  (check-true ((->path "foo.TXT") . has-ext? . 'txt))
  (check-true (has-ext? foo.bar.txt-path 'txt))
  (check-false (foo.bar.txt-path . has-ext? . 'doc)) ; wrong extension
+ (check-exn exn:fail:contract? (λ () (has-ext? #f "foo")))
+ (check-exn exn:fail:contract? (λ () (has-ext? "foo" #f)))
  
  (check-equal? (get-ext (->path "foo.txt")) "txt")
  (check-false (get-ext "foo"))
  (check-false (get-ext ".foo"))
+ (check-exn exn:fail:contract? (λ () (get-ext #f)))
  
  (check-equal? (add-ext (string->path "foo") "txt") (string->path "foo.txt"))
+ (check-exn exn:fail:contract? (λ () (add-ext "foo" #f)))
+ (check-exn exn:fail:contract? (λ () (add-ext #f "foo" )))
+ 
  (check-equal? (remove-ext foo-path) foo-path)
  (check-equal? (remove-ext (->path ".foo.txt")) (->path ".foo"))
  (check-equal? (remove-ext foo.txt-path) foo-path)