diff --git a/h2o-py/h2o/frame.py b/h2o-py/h2o/frame.py
index 76e8b08ad9a5..16de048d0716 100644
--- a/h2o-py/h2o/frame.py
+++ b/h2o-py/h2o/frame.py
@@ -1980,8 +1980,8 @@ def as_data_frame(self, use_pandas=True, header=True, multi_thread=False):
                         os.remove(fileName)
                         os.rmdir(tmpdir)
                 elif not(can_use_datatable()):
-                    warnings("multi_thread mode can only be used when you have datatable "
-                             "installed.")                   
+                    warnings.warn("multi_thread mode can only be used when you have datatable "
+                             "installed.  Defaults to single-thread operation.")                   
             return pandas.read_csv(StringIO(self.get_frame_data()), low_memory=False, skip_blank_lines=False)
         from h2o.utils.csv.readers import reader
         frame = [row for row in reader(StringIO(self.get_frame_data()))]
diff --git a/h2o-py/tests/pyunit_utils/utilsPY.py b/h2o-py/tests/pyunit_utils/utilsPY.py
index df31448a511f..8254bd0e1a2a 100644
--- a/h2o-py/tests/pyunit_utils/utilsPY.py
+++ b/h2o-py/tests/pyunit_utils/utilsPY.py
@@ -595,6 +595,9 @@ def javamunge(assembly, pojoname, test, compile_only=False):
 def install(package):
     subprocess.check_call([sys.executable, "-m", "pip", "install", package])
     
+def uninstall(package):
+    subprocess.check_call([sys.executable, "-m", "pip", "uninstall", "-y", package])
+        
 def locate(path):
     """
     Search for a relative path and turn it into an absolute path.
diff --git a/h2o-py/tests/testdir_misc/pyunit_gh_15861_no_datatable.py b/h2o-py/tests/testdir_misc/pyunit_gh_15861_no_datatable.py
new file mode 100644
index 000000000000..1acd623ea669
--- /dev/null
+++ b/h2o-py/tests/testdir_misc/pyunit_gh_15861_no_datatable.py
@@ -0,0 +1,28 @@
+import sys
+sys.path.insert(1,"../../")
+import h2o
+from tests import pyunit_utils
+from h2o.utils.shared_utils import (can_use_datatable)
+    
+def test_datatable_without_datatable():
+    delTable = False
+    if can_use_datatable():
+        delTable = True
+        pyunit_utils.uninstall("datatable")
+        
+    try:   
+        # should run to completion
+        with pyunit_utils.catch_warnings() as ws:
+            h2oFrame = h2o.import_file(pyunit_utils.locate("bigdata/laptop/jira/PUBDEV_5266_merge_with_string_columns/PUBDEV_5266_f1.csv"))
+            new_frame = h2oFrame.as_data_frame(multi_thread=True) 
+            assert "multi_thread mode can only be used when you have datatable installed.  Defaults to single-thread " \
+                "operation." in str(ws[0].message)
+    finally:
+        # re-install datatable before quitting.     
+        if delTable:
+            pyunit_utils.install("datatable")
+
+if __name__ == "__main__":
+    pyunit_utils.standalone_test(test_datatable_without_datatable)
+else:
+    test_datatable_without_datatable()