diff --git a/spring-test/src/main/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilder.java b/spring-test/src/main/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilder.java
index c5b527b4adab..51be1286e5d7 100644
--- a/spring-test/src/main/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilder.java
+++ b/spring-test/src/main/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilder.java
@@ -294,7 +294,8 @@ public MockHttpServletRequestBuilder contentType(MediaType contentType) {
 	 * @since 4.1.2
 	 */
 	public MockHttpServletRequestBuilder contentType(String contentType) {
-		this.contentType = MediaType.parseMediaType(contentType).toString();
+		Assert.notNull(contentType, "'contentType' must not be null");
+		this.contentType = contentType;
 		return this;
 	}
 
@@ -314,11 +315,9 @@ public MockHttpServletRequestBuilder accept(MediaType... mediaTypes) {
 	 */
 	public MockHttpServletRequestBuilder accept(String... mediaTypes) {
 		Assert.notEmpty(mediaTypes, "'mediaTypes' must not be empty");
-		List<MediaType> result = new ArrayList<>(mediaTypes.length);
-		for (String mediaType : mediaTypes) {
-			result.add(MediaType.parseMediaType(mediaType));
-		}
-		this.headers.set("Accept", MediaType.toString(result));
+		List<String> result = new ArrayList<>(mediaTypes.length);
+		result.addAll(Arrays.asList(mediaTypes));
+		this.headers.set("Accept", String.join(", ", result));
 		return this;
 	}
 
@@ -712,7 +711,7 @@ public final MockHttpServletRequest buildRequest(ServletContext servletContext)
 
 		if (this.content != null && this.content.length > 0) {
 			String requestContentType = request.getContentType();
-			if (requestContentType != null) {
+			if (requestContentType != null && isValidContentType(requestContentType)) {
 				MediaType mediaType = MediaType.parseMediaType(requestContentType);
 				if (MediaType.APPLICATION_FORM_URLENCODED.includes(mediaType)) {
 					addRequestParams(request, parseFormData(mediaType));
@@ -742,6 +741,26 @@ public final MockHttpServletRequest buildRequest(ServletContext servletContext)
 		return request;
 	}
 
+	/**
+	 * Some validation checks to find out if processing of a string value make sense.
+	 */
+	private boolean isValidContentType(String mimeType) {
+		if (!StringUtils.hasLength(mimeType)) {
+			return false;
+		}
+
+		int index = mimeType.indexOf(';');
+		String fullType = (index >= 0 ? mimeType.substring(0, index) : mimeType).trim();
+		if (fullType.isEmpty()) {
+			return false;
+		}
+		int subIndex = fullType.indexOf('/');
+		if (subIndex == -1) {
+			return false;
+		}
+		return subIndex != fullType.length() - 1;
+	}
+
 	/**
 	 * Create a new {@link MockHttpServletRequest} based on the supplied
 	 * {@code ServletContext}.
diff --git a/spring-test/src/test/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilderTests.java b/spring-test/src/test/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilderTests.java
index ae111d7877b6..3479d385e622 100644
--- a/spring-test/src/test/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilderTests.java
+++ b/spring-test/src/test/java/org/springframework/test/web/servlet/request/MockHttpServletRequestBuilderTests.java
@@ -29,6 +29,8 @@
 import java.util.Locale;
 import java.util.Map;
 
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
 import javax.servlet.ServletContext;
 import javax.servlet.http.Cookie;
 
@@ -337,6 +339,21 @@ public void acceptHeader() {
 		assertThat(result.get(1).toString()).isEqualTo("application/xml");
 	}
 
+	@Test
+	public void anyAcceptMultipleContentTypeViaStringArray() {
+		this.builder.accept("any", "any2");
+
+		MockHttpServletRequest request = this.builder.buildRequest(this.servletContext);
+		List<String> accept = Collections.list(request.getHeaders("Accept"));
+		List<String> result = Stream.of(accept.get(0).split(","))
+				.map(String::trim)
+				.collect(Collectors.toList());
+
+		assertThat(result.get(0)).isEqualTo("any");
+		assertThat(result).hasSize(2);
+		assertThat(result.get(1)).isEqualTo("any2");
+	}
+
 	@Test
 	public void contentType() {
 		this.builder.contentType(MediaType.TEXT_HTML);
@@ -363,6 +380,19 @@ public void contentTypeViaString() {
 		assertThat(contentTypes.get(0)).isEqualTo("text/html");
 	}
 
+	@Test
+	public void anyContentTypeViaString() {
+		this.builder.contentType("any");
+
+		MockHttpServletRequest request = this.builder.buildRequest(this.servletContext);
+		String contentType = request.getContentType();
+		List<String> contentTypes = Collections.list(request.getHeaders("Content-Type"));
+
+		assertThat(contentType).isEqualTo("any");
+		assertThat(contentTypes).hasSize(1);
+		assertThat(contentTypes.get(0)).isEqualTo("any");
+	}
+
 	@Test  // SPR-11308
 	public void contentTypeViaHeader() {
 		this.builder.header("Content-Type", MediaType.TEXT_HTML_VALUE);
@@ -372,6 +402,16 @@ public void contentTypeViaHeader() {
 		assertThat(contentType).isEqualTo("text/html");
 	}
 
+	@Test  // SPR-17643
+	public void invalidContentTypeViaHeader() {
+		this.builder.header("Content-Type", "yaml");
+		this.builder.content("some content");
+		MockHttpServletRequest request = this.builder.buildRequest(this.servletContext);
+		String contentType = request.getContentType();
+
+		assertThat(contentType).isEqualTo("yaml");
+	}
+
 	@Test  // SPR-11308
 	public void contentTypeViaMultipleHeaderValues() {
 		this.builder.header("Content-Type", MediaType.TEXT_HTML_VALUE, MediaType.ALL_VALUE);