diff --git a/django/templatetags/i18n.py b/django/templatetags/i18n.py
index d0fda0797e..93023f3109 100644
--- a/django/templatetags/i18n.py
+++ b/django/templatetags/i18n.py
@@ -1,3 +1,5 @@
+from decimal import Decimal
+
 from django.conf import settings
 from django.template import Library, Node, TemplateSyntaxError, Variable
 from django.template.base import TokenType, render_value_in_context
@@ -135,6 +137,11 @@ class BlockTranslateNode(Node):
         singular, vars = self.render_token_list(self.singular)
         if self.plural and self.countervar and self.counter:
             count = self.counter.resolve(context)
+            if not isinstance(count, (Decimal, float, int)):
+                raise TemplateSyntaxError(
+                    "%r argument to %r tag must be a number."
+                    % (self.countervar, self.tag_name)
+                )
             context[self.countervar] = count
             plural, plural_vars = self.render_token_list(self.plural)
             if message_context:
diff --git a/tests/template_tests/syntax_tests/i18n/test_blocktranslate.py b/tests/template_tests/syntax_tests/i18n/test_blocktranslate.py
index 8edacd87e2..5079824b30 100644
--- a/tests/template_tests/syntax_tests/i18n/test_blocktranslate.py
+++ b/tests/template_tests/syntax_tests/i18n/test_blocktranslate.py
@@ -298,6 +298,15 @@ class I18nBlockTransTagTests(SimpleTestCase):
         with self.assertRaisesMessage(TemplateSyntaxError, msg):
             self.engine.render_to_string('template', {'a': [1, 2, 3]})
 
+    @setup({'template': (
+        '{% load i18n %}{% blocktranslate count counter=num %}{{ counter }}'
+        '{% plural %}{{ counter }}{% endblocktranslate %}'
+    )})
+    def test_count_not_number(self, tag_name):
+        msg = "'counter' argument to '{}' tag must be a number.".format(tag_name)
+        with self.assertRaisesMessage(TemplateSyntaxError, msg):
+            self.engine.render_to_string('template', {'num': '1'})
+
     @setup({'template': (
         '{% load i18n %}{% blocktranslate count count=var|length %}'
         'There is {{ count }} object. {% block a %} {% endblock %}'