diff --git a/ChangeLog.md b/ChangeLog.md
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -1,5 +1,9 @@
 # Revision history for patch
 
+## 0.0.3.1
+
+* Replace `fromJust` with something easier to debug.
+
 ## 0.0.3.0
 
 * Create `PatchMapWithPatchingMove` variant which supports moves with a patch.
diff --git a/patch.cabal b/patch.cabal
--- a/patch.cabal
+++ b/patch.cabal
@@ -1,5 +1,5 @@
 Name: patch
-Version: 0.0.3.0
+Version: 0.0.3.1
 Synopsis: Infrastructure for writing patches which act on other types.
 Description:
   In this library, a patch is something which can be applied, analogous to a
diff --git a/src/Data/Patch/MapWithMove.hs b/src/Data/Patch/MapWithMove.hs
--- a/src/Data/Patch/MapWithMove.hs
+++ b/src/Data/Patch/MapWithMove.hs
@@ -202,7 +202,9 @@
                       putRemainingKeys $ Set.delete k fromKs
                       return $ NodeInfo (From_Move k) $ Just undefined -- There's an existing value, and it's here, so no patch necessary
                     else do
-                      (fromK, remainingKeys) <- return . fromMaybe (error "patchThatChangesMap: impossible: fromKs was empty") $ Set.minView fromKs -- There's an existing value, but it's not here; move it here
+                      (fromK, remainingKeys) <- return $
+                        fromMaybe (error "PatchMapWithMove.patchThatChangesMap: impossible: fromKs was empty") $
+                        Set.minView fromKs -- There's an existing value, but it's not here; move it here
                       putRemainingKeys remainingKeys
                       return $ NodeInfo (From_Move fromK) $ Just undefined
           Map.traverseWithKey f newByIndex
diff --git a/src/Data/Patch/MapWithPatchingMove.hs b/src/Data/Patch/MapWithPatchingMove.hs
--- a/src/Data/Patch/MapWithPatchingMove.hs
+++ b/src/Data/Patch/MapWithPatchingMove.hs
@@ -268,7 +268,9 @@
                       putRemainingKeys $ Set.delete k fromKs
                       return $ NodeInfo (From_Move k mempty) $ Just undefined -- There's an existing value, and it's here, so no patch necessary
                     else do
-                      (fromK, remainingKeys) <- return . fromJust $ Set.minView fromKs -- There's an existing value, but it's not here; move it here
+                      (fromK, remainingKeys) <- return $
+                        fromMaybe (error "PatchMapWithPatchingMove.patchThatChangesMap: impossible: fromKs was empty") $
+                        Set.minView fromKs -- There's an existing value, but it's not here; move it here
                       putRemainingKeys remainingKeys
                       return $ NodeInfo (From_Move fromK mempty) $ Just undefined
           Map.traverseWithKey f newByIndex