diff --git a/guix/gexp.scm b/guix/gexp.scm
index 1b8e43e99..80d8f735b 100644
--- a/guix/gexp.scm
+++ b/guix/gexp.scm
@@ -26,6 +26,8 @@
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-9 gnu)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
   #:use-module (ice-9 match)
   #:export (gexp
             gexp?
@@ -84,7 +86,13 @@
             gexp-compiler?
             lower-object
 
-            lower-inputs))
+            lower-inputs
+
+            &gexp-error
+            gexp-error?
+            &gexp-input-error
+            gexp-input-error?
+            gexp-error-invalid-input))
 
 ;;; Commentary:
 ;;;
@@ -140,6 +148,14 @@
   (lower      gexp-compiler-lower)
   (expand     gexp-compiler-expand))              ;#f | DRV -> sexp
 
+(define-condition-type &gexp-error &error
+  gexp-error?)
+
+(define-condition-type &gexp-input-error &gexp-error
+  gexp-input-error?
+  (input gexp-error-invalid-input))
+
+
 (define %gexp-compilers
   ;; 'eq?' mapping of record type descriptor to <gexp-compiler>.
   (make-hash-table 20))
@@ -177,8 +193,11 @@ procedure to expand it; otherwise return #f."
 corresponding to OBJ for SYSTEM, cross-compiling for TARGET if TARGET is true.
 OBJ must be an object that has an associated gexp compiler, such as a
 <package>."
-  (let ((lower (lookup-compiler obj)))
-    (lower obj system target)))
+  (match (lookup-compiler obj)
+    (#f
+     (raise (condition (&gexp-input-error (input obj)))))
+    (lower
+     (lower obj system target))))
 
 (define-syntax define-gexp-compiler
   (syntax-rules (=> compiler expander)
diff --git a/guix/packages.scm b/guix/packages.scm
index 61171b834..d865dd5ad 100644
--- a/guix/packages.scm
+++ b/guix/packages.scm
@@ -31,7 +31,6 @@
   #:use-module (guix memoization)
   #:use-module (guix build-system)
   #:use-module (guix search-paths)
-  #:use-module (guix gexp)
   #:use-module (guix sets)
   #:use-module (ice-9 match)
   #:use-module (ice-9 vlist)
@@ -1226,4 +1225,7 @@ outside of the store) or SOURCE itself (if SOURCE is already a store item.)"
         ((? string? file)
          (add-to-store store (basename file) #t "sha256" file))
         (_
+         ;; XXX: 'lower' can throw '&gexp-input-error'.  From a UI viewpoint,
+         ;; this is better than a wrong-type-arg error, but it's not ideal
+         ;; because gexps don't have source location information yet.
          (lower store source system))))))
diff --git a/guix/ui.scm b/guix/ui.scm
index 345bf490b..b3c94795f 100644
--- a/guix/ui.scm
+++ b/guix/ui.scm
@@ -26,6 +26,7 @@
 ;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
 
 (define-module (guix ui)
+  #:use-module (guix gexp)
   #:use-module (guix utils)
   #:use-module (guix store)
   #:use-module (guix config)
@@ -448,6 +449,10 @@ interpreted."
                       (location->string loc)
                       (package-full-name package)
                       (build-system-name system))))
+            ((gexp-input-error? c)
+             (let ((input (package-error-invalid-input c)))
+               (leave (_ "~s: invalid G-expression input~%")
+                      (gexp-error-invalid-input c))))
             ((profile-not-found-error? c)
              (leave (_ "profile '~a' does not exist~%")
                     (profile-error-profile c)))