diff --git a/repo2docker/buildpacks/base.py b/repo2docker/buildpacks/base.py
index dedd915f..ba50356b 100644
--- a/repo2docker/buildpacks/base.py
+++ b/repo2docker/buildpacks/base.py
@@ -525,6 +525,9 @@ class BuildPack:
 
         # If you work on this bit of code check the corresponding code in
         # buildpacks/docker.py where it is duplicated
+        if not isinstance(memory_limit, int):
+            raise ValueError("The memory limit has to be specified as an"
+                             "integer but is '{}'".format(type(memory_limit)))
         limits = {}
         if memory_limit:
             # We'd like to always disable swap but all we can do is set the
diff --git a/repo2docker/buildpacks/docker.py b/repo2docker/buildpacks/docker.py
index a9da371d..208807ed 100644
--- a/repo2docker/buildpacks/docker.py
+++ b/repo2docker/buildpacks/docker.py
@@ -23,6 +23,9 @@ class DockerBuildPack(BuildPack):
         """Build a Docker image based on the Dockerfile in the source repo."""
         # If you work on this bit of code check the corresponding code in
         # buildpacks/base.py where it is duplicated
+        if not isinstance(memory_limit, int):
+            raise ValueError("The memory limit has to be specified as an"
+                             "integer but is '{}'".format(type(memory_limit)))
         limits = {}
         if memory_limit:
             # We'd like to always disable swap but all we can do is set the
diff --git a/tests/unit/test_memlimit.py b/tests/unit/test_memlimit.py
index f636da7c..1d3f88ba 100644
--- a/tests/unit/test_memlimit.py
+++ b/tests/unit/test_memlimit.py
@@ -10,9 +10,14 @@ import os
 import shutil
 import time
 
+from unittest.mock import MagicMock
+
+import docker
+
 import pytest
 
 from repo2docker.app import Repo2Docker
+from repo2docker.buildpacks import BaseImage, DockerBuildPack
 
 
 basedir = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
@@ -82,3 +87,17 @@ def test_memlimit_same_postbuild():
             file_contents.append(f.read())
     # Make sure they're all the same
     assert len(set(file_contents)) == 1
+
+
+@pytest.mark.parametrize('BuildPack', [BaseImage, DockerBuildPack])
+def test_memlimit_argument_type(BuildPack):
+    # check that an exception is raised when the memory limit isn't an int
+    fake_log_value = {'stream': 'fake'}
+    fake_client = MagicMock(spec=docker.APIClient)
+    fake_client.build.return_value = iter([fake_log_value])
+
+    with pytest.raises(ValueError) as exc_info:
+        for line in BuildPack().build(fake_client, 'image-2', "10Gi", {}, [], {}):
+            pass
+
+        assert "The memory limit has to be specified as an" in str(exc_info.value)