diff --git a/pkgs/applications/misc/buku/default.nix b/pkgs/applications/misc/buku/default.nix
index 5db3e7d0c8c..ac65341d498 100644
--- a/pkgs/applications/misc/buku/default.nix
+++ b/pkgs/applications/misc/buku/default.nix
@@ -18,7 +18,7 @@ with python3.pkgs; buildPythonApplication rec {
     pylint
     flake8
     pyyaml
-    mypy_extensions
+    mypy-extensions
   ];
 
   propagatedBuildInputs = [
diff --git a/pkgs/development/python-modules/mypy/default.nix b/pkgs/development/python-modules/mypy/default.nix
index 9d55bf33f9b..6c64580b2cf 100644
--- a/pkgs/development/python-modules/mypy/default.nix
+++ b/pkgs/development/python-modules/mypy/default.nix
@@ -1,5 +1,7 @@
 { stdenv, fetchPypi, buildPythonPackage, typed-ast, psutil, isPy3k
-,mypy_extensions }:
+, mypy-extensions
+, typing-extensions
+}:
 
 buildPythonPackage rec {
   pname = "mypy";
@@ -15,7 +17,7 @@ buildPythonPackage rec {
 
   disabled = !isPy3k;
 
-  propagatedBuildInputs = [ typed-ast psutil mypy_extensions ];
+  propagatedBuildInputs = [ typed-ast psutil mypy-extensions typing-extensions ];
 
   meta = with stdenv.lib; {
     description = "Optional static typing for Python";
diff --git a/pkgs/development/python-modules/mypy/extensions.nix b/pkgs/development/python-modules/mypy/extensions.nix
index 5cbb01a8c54..b3bb68a53b7 100644
--- a/pkgs/development/python-modules/mypy/extensions.nix
+++ b/pkgs/development/python-modules/mypy/extensions.nix
@@ -1,14 +1,15 @@
 { stdenv, fetchPypi, buildPythonPackage, typing, pythonOlder }:
 
 buildPythonPackage rec {
-  pname = "mypy_extensions";
+  pname = "mypy-extensions";
   version = "0.4.3";
 
   # Tests not included in pip package.
   doCheck = false;
 
   src = fetchPypi {
-    inherit pname version;
+    inherit version;
+    pname = "mypy_extensions";
     sha256 = "2d82818f5bb3e369420cb3c4060a7970edba416647068eb4c5343488a6c604a8";
   };
 
diff --git a/pkgs/development/python-modules/pyannotate/default.nix b/pkgs/development/python-modules/pyannotate/default.nix
index a09463a76d3..06dcdf83bcc 100644
--- a/pkgs/development/python-modules/pyannotate/default.nix
+++ b/pkgs/development/python-modules/pyannotate/default.nix
@@ -3,7 +3,7 @@
 , fetchPypi
 , pythonOlder
 , six
-, mypy_extensions
+, mypy-extensions
 , typing
 , pytest
 }:
@@ -18,7 +18,7 @@ buildPythonPackage rec {
   };
 
   checkInputs = [ pytest ];
-  propagatedBuildInputs = [ six mypy_extensions ]
+  propagatedBuildInputs = [ six mypy-extensions ]
     ++ stdenv.lib.optionals (pythonOlder "3.5") [ typing ];
 
   checkPhase = ''
diff --git a/pkgs/top-level/python-packages.nix b/pkgs/top-level/python-packages.nix
index b8896945eaf..044a5fda7c1 100644
--- a/pkgs/top-level/python-packages.nix
+++ b/pkgs/top-level/python-packages.nix
@@ -3913,7 +3913,7 @@ in {
 
   mypy = callPackage ../development/python-modules/mypy { };
 
-  mypy_extensions = callPackage ../development/python-modules/mypy/extensions.nix { };
+  mypy-extensions = callPackage ../development/python-modules/mypy/extensions.nix { };
 
   mypy-protobuf = callPackage ../development/python-modules/mypy-protobuf { };