diff --git a/fsspec/implementations/memory.py b/fsspec/implementations/memory.py
index 001b6a31e..f003a7db6 100644
--- a/fsspec/implementations/memory.py
+++ b/fsspec/implementations/memory.py
@@ -15,7 +15,7 @@ class MemoryFileSystem(AbstractFileSystem):
     protocol = "memory"
     root_marker = ""
 
-    def ls(self, path, detail=False):
+    def ls(self, path, detail=False, **kwargs):
         if path in self.store:
             # there is a key with this exact name, but could also be directory
             out = [
@@ -127,7 +127,10 @@ def copy(self, path1, path2, **kwargs):
         self.store[path2] = MemoryFile(self, path2, self.store[path1].getbuffer())
 
     def cat(self, path):
-        return self.store[path].getvalue()
+        try:
+            return self.store[path].getvalue()
+        except KeyError:
+            raise FileNotFoundError(path)
 
     def _rm(self, path):
         del self.store[path]
diff --git a/fsspec/mapping.py b/fsspec/mapping.py
index e8b45e62f..2f27454da 100644
--- a/fsspec/mapping.py
+++ b/fsspec/mapping.py
@@ -71,10 +71,10 @@ def _str_to_key(self, s):
 
     def __getitem__(self, key, default=None):
         """Retrieve data"""
-        key = self._key_to_str(key)
+        k = self._key_to_str(key)
         try:
-            result = self.fs.cat(key)
-        except:  # noqa: E722
+            result = self.fs.cat(k)
+        except FileNotFoundError:
             if default is not None:
                 return default
             raise KeyError(key)
diff --git a/tox.ini b/tox.ini
index 63dfc2449..e11080134 100644
--- a/tox.ini
+++ b/tox.ini
@@ -21,6 +21,7 @@ conda_deps=
     pytest
     pytest-cov
     fusepy==3.0.1
+    msgpack-python<1.0.0
 deps=
     hadoop-test-cluster==0.1.0