diff --git a/CHANGES b/CHANGES
index 9e473bde1..ad1436925 100644
--- a/CHANGES
+++ b/CHANGES
@@ -18,6 +18,7 @@ Bugs fixed
 
 * #1922: html search: Upper characters problem in French
 * #4412: Updated jQuery version from 3.1.0 to 3.2.1
+* #4438: math: math with labels with whitespace cause html error
 
 Testing
 --------
diff --git a/sphinx/ext/mathbase.py b/sphinx/ext/mathbase.py
index 549ca30cd..8370c923e 100644
--- a/sphinx/ext/mathbase.py
+++ b/sphinx/ext/mathbase.py
@@ -10,6 +10,7 @@
 """
 
 from docutils import nodes, utils
+from docutils.nodes import make_id
 from docutils.parsers.rst import Directive, directives
 
 from sphinx.roles import XRefRole
@@ -51,7 +52,8 @@ class MathDomain(Domain):
     label = 'mathematics'
 
     initial_data = {
-        'objects': {},  # labelid -> (docname, eqno)
+        'nameids': {},  # label -> equation ID
+        'objects': {},  # equation ID -> (docname, eqno)
     }  # type: Dict[unicode, Dict[unicode, Tuple[unicode, int]]]
     dangling_warnings = {
         'eq': 'equation not found: %(target)s',
@@ -59,9 +61,9 @@ class MathDomain(Domain):
 
     def clear_doc(self, docname):
         # type: (unicode) -> None
-        for labelid, (doc, eqno) in list(self.data['objects'].items()):
+        for equation_id, (doc, eqno) in list(self.data['objects'].items()):
             if doc == docname:
-                del self.data['objects'][labelid]
+                del self.data['objects'][equation_id]
 
     def merge_domaindata(self, docnames, otherdata):
         # type: (Iterable[unicode], Dict) -> None
@@ -81,8 +83,8 @@ class MathDomain(Domain):
                 return newnode
             else:
                 title = nodes.Text("(%d)" % number)
-                return make_refnode(builder, fromdocname, docname,
-                                    "equation-" + target, title)
+                node_id = make_id('equation-%s' % target)
+                return make_refnode(builder, fromdocname, docname, node_id, title)
         else:
             return None
 
@@ -226,7 +228,8 @@ class MathDirective(Directive):
             node['number'] = eqno
 
             # add target node
-            target = nodes.target('', '', ids=['equation-' + node['label']])
+            node_id = make_id('equation-%s' % node['label'])
+            target = nodes.target('', '', ids=[node_id])
             self.state.document.note_explicit_target(target)
             ret.insert(0, target)
         except UserWarning as exc:
diff --git a/tests/test_ext_math.py b/tests/test_ext_math.py
index 2b8a68f9c..8ecf7ca4d 100644
--- a/tests/test_ext_math.py
+++ b/tests/test_ext_math.py
@@ -27,9 +27,9 @@ def test_jsmath(app, status, warning):
     assert (u'<span class="eqno">(1)<a class="headerlink" href="#equation-foo" '
             u'title="Permalink to this equation">\xb6</a></span>'
             u'<div class="math" id="equation-foo">\ne^{i\\pi} = 1</div>' in content)
-    assert (u'<span class="eqno">(2)<a class="headerlink" href="#equation-math:0" '
+    assert (u'<span class="eqno">(2)<a class="headerlink" href="#equation-math-0" '
             u'title="Permalink to this equation">\xb6</a></span>'
-            u'<div class="math" id="equation-math:0">\n'
+            u'<div class="math" id="equation-math-0">\n'
             u'e^{ix} = \\cos x + i\\sin x</div>' in content)
     assert '<div class="math">\nn \\in \\mathbb N</div>' in content
     assert '<div class="math">\na + 1 &lt; b</div>' in content
@@ -85,7 +85,7 @@ def test_math_number_all_mathjax(app, status, warning):
     app.builder.build_all()
 
     content = (app.outdir / 'index.html').text()
-    html = (r'<div class="math" id="equation-index:0">\s*'
+    html = (r'<div class="math" id="equation-index-0">\s*'
             r'<span class="eqno">\(1\)<a .*>\xb6</a></span>\\\[a\^2\+b\^2=c\^2\\\]</div>')
     assert re.search(html, content, re.S)