diff --git a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletRequest.java b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletRequest.java
index 7592528b62..f1d54c4d3a 100644
--- a/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletRequest.java
+++ b/spring-test/src/main/java/org/springframework/mock/web/MockHttpServletRequest.java
@@ -488,7 +488,8 @@ public class MockHttpServletRequest implements HttpServletRequest {
 			}
 			catch (IllegalArgumentException ex) {
 				// Try to get charset value anyway
-				int charsetIndex = contentType.toLowerCase().indexOf(CHARSET_PREFIX);
+				contentType = contentType.toLowerCase();
+				int charsetIndex = contentType.indexOf(CHARSET_PREFIX);
 				if (charsetIndex != -1) {
 					this.characterEncoding = contentType.substring(charsetIndex + CHARSET_PREFIX.length());
 				}
diff --git a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java
index 3015d43c4f..f2713d2618 100644
--- a/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java
+++ b/spring-test/src/test/java/org/springframework/mock/web/MockHttpServletRequestTests.java
@@ -185,6 +185,15 @@ class MockHttpServletRequestTests {
 		assertThat(request.getCharacterEncoding()).isEqualTo("UTF-8");
 	}
 
+	@Test // gh-29255
+	void setContentTypeInvalidWithNonAsciiCharacterAndCharset() {
+		String contentType = "İcharset=";
+		request.addHeader(HttpHeaders.CONTENT_TYPE, contentType);
+		assertThat(request.getContentType()).isEqualTo(contentType);
+		assertThat(request.getHeader(HttpHeaders.CONTENT_TYPE)).isEqualTo(contentType);
+		assertThat(request.getCharacterEncoding()).isEqualTo("");
+	}
+
 	@Test
 	void contentTypeHeader() {
 		String contentType = "test/plain";
diff --git a/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletRequest.java b/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletRequest.java
index 74e1982bb3..065c37bf3a 100644
--- a/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletRequest.java
+++ b/spring-web/src/testFixtures/java/org/springframework/web/testfixture/servlet/MockHttpServletRequest.java
@@ -488,7 +488,8 @@ public class MockHttpServletRequest implements HttpServletRequest {
 			}
 			catch (IllegalArgumentException ex) {
 				// Try to get charset value anyway
-				int charsetIndex = contentType.toLowerCase().indexOf(CHARSET_PREFIX);
+				contentType = contentType.toLowerCase();
+				int charsetIndex = contentType.indexOf(CHARSET_PREFIX);
 				if (charsetIndex != -1) {
 					this.characterEncoding = contentType.substring(charsetIndex + CHARSET_PREFIX.length());
 				}