diff --git a/CHANGES.md b/CHANGES.md
index 707592c2..839d1a14 100644
--- a/CHANGES.md
+++ b/CHANGES.md
@@ -2,7 +2,7 @@
 
 ## python-markdown2 2.5.6 (not yet released)
 
-(nothing yet)
+- [pull #687] Fix AssertionError hashing HTML blocks spread over multiple lines (#686)
 
 
 ## python-markdown2 2.5.5
diff --git a/lib/markdown2.py b/lib/markdown2.py
index b831896f..14ae415c 100755
--- a/lib/markdown2.py
+++ b/lib/markdown2.py
@@ -922,7 +922,7 @@ def _hash_html_block_sub(
                 tag = None
 
         if not tag:
-            m = re.match(r'.*?<(\S).*?\s*>', html)
+            m = re.match(r'.*?<(\S).*?\s*>', html, re.S)
             # tag shouldn't be none but make the assertion for type checker
             assert m is not None
             tag = m.group(1)
diff --git a/test/tm-cases/hash_html_blocks_issue686.html b/test/tm-cases/hash_html_blocks_issue686.html
new file mode 100644
index 00000000..a6398989
--- /dev/null
+++ b/test/tm-cases/hash_html_blocks_issue686.html
@@ -0,0 +1,3 @@
+<p
+class="abc">test
+</p>
diff --git a/test/tm-cases/hash_html_blocks_issue686.tags b/test/tm-cases/hash_html_blocks_issue686.tags
new file mode 100644
index 00000000..7bd1a023
--- /dev/null
+++ b/test/tm-cases/hash_html_blocks_issue686.tags
@@ -0,0 +1 @@
+issue686 assertionerror
\ No newline at end of file
diff --git a/test/tm-cases/hash_html_blocks_issue686.text b/test/tm-cases/hash_html_blocks_issue686.text
new file mode 100644
index 00000000..a6398989
--- /dev/null
+++ b/test/tm-cases/hash_html_blocks_issue686.text
@@ -0,0 +1,3 @@
+<p
+class="abc">test
+</p>