diff --git a/bin/is b/bin/is
index caaab5ad521889b91cf5f53720da0cc192f66d81..67006526f341e99fe7776d34c2d05850985e0b8d 100755
--- a/bin/is
+++ b/bin/is
@@ -81,7 +81,7 @@ def c_add(parser, args):
 
 def c_build(parser, args):
     '''
-    Build an image source
+    Build an image source in current directory
     '''
     # build start time
     t0 = time.time()
diff --git a/installsystems/image.py b/installsystems/image.py
index d6fd3befef37cca6a949edc952d1e2400c2f72ab..c1a5f2281505d67895d7e3aef961c6d81e76aa04 100644
--- a/installsystems/image.py
+++ b/installsystems/image.py
@@ -148,13 +148,13 @@ class SourceImage(Image):
         '''
         for d in (self.base_path, self.parser_path, self.setup_path, self.payload_path):
             if not os.path.exists(d):
-                raise Exception("Missing directory: %s" % d)
+                raise Exception("Invalid source image: %s is missing" % d)
             if not os.path.isdir(d):
-                raise Exception("Not a directory: %s" % d)
+                raise Exception("Invalid source image: %s is not a directory" % d)
             if not os.access(d, os.R_OK|os.X_OK):
-                raise Exception("Unable to access to %s" % d)
+                raise Exception("Invalid source image: unable to access to %s" % d)
         if not os.path.exists(os.path.join(self.base_path, "description")):
-            raise Exception("No description file")
+            raise Exception("Invalid source image: no description file")
 
     def build(self, force=False, force_payload=False, check=True):
         '''