diff --git a/cfbs/build.py b/cfbs/build.py
index cf33d7ce..7c295fbd 100644
--- a/cfbs/build.py
+++ b/cfbs/build.py
@@ -10,6 +10,7 @@
 elsewhere, like validation and downloading modules.
 """
 
+import json
 import os
 import logging as log
 import shutil
@@ -485,7 +486,12 @@ def perform_build(config: CFBSConfig, diffs_filename=None) -> int:
     assert os.path.isdir("./out/masterfiles/")
     shutil.copyfile("./cfbs.json", "./out/masterfiles/cfbs.json")
     if os.path.isfile("out/masterfiles/def.json"):
-        pretty_file("out/masterfiles/def.json")
+        try:
+            pretty_file("out/masterfiles/def.json")
+        except json.decoder.JSONDecodeError as e:
+            raise CFBSExitError(
+                "Error parsing JSON in 'out/masterfiles/def.json': %s" % e
+            )
     print("")
     print("Generating tarball...")
     sh("( cd out/ && tar -czf masterfiles.tgz masterfiles )")
diff --git a/cfbs/commands.py b/cfbs/commands.py
index fbeb277c..63491d92 100644
--- a/cfbs/commands.py
+++ b/cfbs/commands.py
@@ -176,7 +176,7 @@ def pretty_command(filenames: List[str], check: bool, keep_order: bool):
         except FileNotFoundError:
             raise CFBSExitError("File '%s' not found" % f)
         except json.decoder.JSONDecodeError as ex:
-            raise CFBSExitError("Error reading json file '{}': {}".format(f, ex))
+            raise CFBSExitError("Error parsing JSON in '%s': %s" % (f, ex))
     if check:
         print("Would reformat %d file(s)" % num_files)
         return 1 if num_files > 0 else 0