diff --git a/lib/test.js b/lib/test.js
index f8300881..7f716535 100644
--- a/lib/test.js
+++ b/lib/test.js
@@ -275,7 +275,11 @@ class Test extends Request {
    */// eslint-disable-next-line class-methods-use-this
   _assertHeader(header, res) {
     const field = header.name;
-    const actual = res.header[field.toLowerCase()];
+    // Use `res.headers` rather than `res.header`: superagent copies
+    // content-type parameters onto the response, so a value such as
+    // `text/csv; header=present` overwrites `res.header` and breaks every
+    // header assertion. `res.headers` is the untouched header map.
+    const actual = res.headers[field.toLowerCase()];
     const fieldExpected = header.value;
 
     if (typeof actual === 'undefined') return new Error('expected "' + field + '" header field');
diff --git a/test/issue-fixes.js b/test/issue-fixes.js
index d84904fe..740f10b7 100644
--- a/test/issue-fixes.js
+++ b/test/issue-fixes.js
@@ -119,4 +119,23 @@ describe('GitHub Issue Fixes', function() {
         });
     });
   });
+
+  describe('Issue #876: "header=" content-type param breaks header matchers', function() {
+    it('should match other headers when content-type has a "header=" parameter', function(done) {
+      const csvApp = express();
+      csvApp.get('/csv', function(req, res) {
+        // `header=present` is a valid text/csv parameter (RFC 7111). superagent
+        // copies content-type params onto the response, clobbering `res.header`.
+        res.setHeader('Content-Type', 'text/csv; header=present');
+        res.setHeader('X-Custom', 'value');
+        res.send('a,b,c');
+      });
+
+      supertest(csvApp)
+        .get('/csv')
+        .expect('X-Custom', 'value')
+        .expect(200)
+        .end(done);
+    });
+  });
 });