diff --git a/moztelemetry/dataset.py b/moztelemetry/dataset.py
index 281bb81..4deac43 100644
--- a/moztelemetry/dataset.py
+++ b/moztelemetry/dataset.py
@@ -6,6 +6,7 @@
 import functools
 import json
 import random
+import re
 import types
 from copy import copy
 from inspect import isfunction
@@ -19,6 +20,7 @@
 from .store import S3Store
 
 MAX_CONCURRENCY = int(cpu_count() * 1.5)
+SANITIZE_PATTERN = re.compile("[^a-zA-Z0-9_.]")
 
 
 def _group_by_size_greedy(obj_list, tot_groups):
@@ -154,12 +156,27 @@ def _apply_selection(self, json_obj):
         return dict((name, path.search(json_obj))
                     for name, path in self.selection_compiled.items())
 
+    def _sanitize_dimension(self, v):
+        """Sanitize the given string by replacing illegal characters
+        with underscores.
+
+        For String conditions, we should pre-sanitize so that users of
+        the `where` function do not need to know about the nuances of how
+        S3 dimensions are sanitized during ingestion.
+
+        See https://github.com/mozilla-services/lua_sandbox_extensions/blob/master/moz_telemetry/io_modules/moz_telemetry/s3.lua#L167
+
+        :param v: a string value that should be sanitized.
+        """
+        return re.sub(SANITIZE_PATTERN, "_", v)
+
     def where(self, **kwargs):
         """Return a new Dataset refined using the given condition
 
         :param kwargs: a map of `dimension` => `condition` to filter the elements
             of the dataset. `condition` can either be an exact value or a
-            callable returning a boolean value.
+            callable returning a boolean value. If `condition` is a value, it is
+            converted to a string, then sanitized.
         """
         clauses = copy(self.clauses)
         for dimension, condition in kwargs.items():
@@ -170,7 +187,7 @@ def where(self, **kwargs):
             if isfunction(condition) or isinstance(condition, functools.partial):
                 clauses[dimension] = condition
             else:
-                clauses[dimension] = functools.partial((lambda x, y: x == y), condition)
+                clauses[dimension] = functools.partial((lambda x, y: x == y), self._sanitize_dimension(str(condition)))
         return Dataset(self.bucket, self.schema, store=self.store,
                        prefix=self.prefix, clauses=clauses, selection=self.selection)
 
diff --git a/tests/test_dataset.py b/tests/test_dataset.py
index 21e5a39..ef14dfe 100644
--- a/tests/test_dataset.py
+++ b/tests/test_dataset.py
@@ -91,11 +91,11 @@ def test_apply_selection():
 
 def test_where_exact_match():
     dataset = Dataset('test-bucket', ['dim1', 'dim2'], prefix='prefix/')
-    new_dataset = dataset.where(dim1='my-value')
+    new_dataset = dataset.where(dim1='myvalue')
     assert new_dataset is not dataset
     assert new_dataset.clauses.keys() == ['dim1']
     condition = new_dataset.clauses['dim1']
-    assert condition('my-value')
+    assert condition('myvalue')
 
 
 def test_where_wrong_dimension():
@@ -169,11 +169,11 @@ def test_scan_multiple_where_params(spark_context):
 
 def test_scan_multiple_params():
     dataset = Dataset('test-bucket', ['dim1', 'dim2'], prefix='prefix/')
-    new_dataset = dataset.where(dim1='my-value')
+    new_dataset = dataset.where(dim1='myvalue')
     assert new_dataset is not dataset
     assert new_dataset.clauses.keys() == ['dim1']
     condition = new_dataset.clauses['dim1']
-    assert condition('my-value')
+    assert condition('myvalue')
 
 
 def test_summaries(spark_context):
@@ -396,3 +396,17 @@ def test_prefix_slash(spark_context):
     summaries_filtered = dataset.where(dim1='dir1').summaries(spark_context)
     assert len(summaries_filtered) == 1
     assert summaries_filtered[0]['key'] == 'a/b/dir1/subdir1/key1'
+
+
+def test_sanitized_dimensions(spark_context):
+    bucket_name = 'test-bucket'
+    store = InMemoryStore(bucket_name)
+    store.store['dir_1/subdir1/key1'] = 'value1'
+    store.store['dir_1/subdir2/key2'] = 'value2'
+    store.store['dir_2/subdir3/key3'] = 'value3'
+    store.store['dir_3/subdir4/key4'] = 'value4'
+
+    dataset = Dataset(bucket_name, ['dim1', 'dim2'], store=store).where(dim1="dir-1")
+
+    summaries = dataset.summaries(spark_context)
+    assert len(summaries) == 2