diff --git a/retrofit/src/main/java/retrofit2/RequestBuilder.java b/retrofit/src/main/java/retrofit2/RequestBuilder.java
index 1303418d3e4442ebc0598c1227a50294249a8b38..3a25b6f53229c21d8fe7ee660d15d00e1b798ded 100644
--- a/retrofit/src/main/java/retrofit2/RequestBuilder.java
+++ b/retrofit/src/main/java/retrofit2/RequestBuilder.java
@@ -75,7 +75,11 @@ final class RequestBuilder {
 
   void addHeader(String name, String value) {
     if ("Content-Type".equalsIgnoreCase(name)) {
-      contentType = MediaType.parse(value);
+      MediaType type = MediaType.parse(value);
+      if (type == null) {
+        throw new IllegalArgumentException("Malformed content type: " + value);
+      }
+      contentType = type;
     } else {
       requestBuilder.addHeader(name, value);
     }
diff --git a/retrofit/src/main/java/retrofit2/ServiceMethod.java b/retrofit/src/main/java/retrofit2/ServiceMethod.java
index facede01fe60adc2f01e9d7c320975f6d9e70a71..26706453a3e826af2abb20114b57fbd2b665604f 100644
--- a/retrofit/src/main/java/retrofit2/ServiceMethod.java
+++ b/retrofit/src/main/java/retrofit2/ServiceMethod.java
@@ -314,7 +314,11 @@ final class ServiceMethod<T> {
         String headerName = header.substring(0, colon);
         String headerValue = header.substring(colon + 1).trim();
         if ("Content-Type".equalsIgnoreCase(headerName)) {
-          contentType = MediaType.parse(headerValue);
+          MediaType type = MediaType.parse(headerValue);
+          if (type == null) {
+            throw methodError("Malformed content type: %s", headerValue);
+          }
+          contentType = type;
         } else {
           builder.add(headerName, headerValue);
         }
diff --git a/retrofit/src/test/java/retrofit2/RequestBuilderTest.java b/retrofit/src/test/java/retrofit2/RequestBuilderTest.java
index 042e08fd15a85b50697271905256450f2455d6f2..1a355feeee351acff66617b6edbbf120d8908960 100644
--- a/retrofit/src/test/java/retrofit2/RequestBuilderTest.java
+++ b/retrofit/src/test/java/retrofit2/RequestBuilderTest.java
@@ -2395,6 +2395,24 @@ public final class RequestBuilderTest {
     assertThat(request.body().contentType().toString()).isEqualTo("text/not-plain");
   }
 
+  @Test public void malformedContentTypeHeaderThrows() {
+    class Example {
+      @POST("/") //
+      @Headers("Content-Type: hello, world!") //
+      Call<ResponseBody> method(@Body RequestBody body) {
+        return null;
+      }
+    }
+    RequestBody body = RequestBody.create(MediaType.parse("text/plain"), "hi");
+    try {
+      buildRequest(Example.class, body);
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertThat(e).hasMessage("Malformed content type: hello, world!\n"
+          + "    for method Example.method");
+    }
+  }
+
   @Test public void contentTypeAnnotationHeaderAddsHeaderWithNoBody() {
     class Example {
       @DELETE("/") //
@@ -2419,6 +2437,22 @@ public final class RequestBuilderTest {
     assertThat(request.body().contentType().toString()).isEqualTo("text/not-plain");
   }
 
+  @Test public void malformedContentTypeParameterThrows() {
+    class Example {
+      @POST("/") //
+      Call<ResponseBody> method(@Header("Content-Type") String contentType, @Body RequestBody body) {
+        return null;
+      }
+    }
+    RequestBody body = RequestBody.create(MediaType.parse("text/plain"), "hi");
+    try {
+      buildRequest(Example.class, "hello, world!", body);
+      fail();
+    } catch (IllegalArgumentException e) {
+      assertThat(e).hasMessage("Malformed content type: hello, world!");
+    }
+  }
+
   @Test public void malformedAnnotationRelativeUrlThrows() {
     class Example {
       @GET("ftp://example.org")