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diff --git a/‎rsocket-core/src/main/java/io/rsocket/metadata/CompositeMetadataFlyweight.java
Lines changed: 3 additions & 1 deletion b/‎rsocket-core/src/main/java/io/rsocket/metadata/CompositeMetadataFlyweight.java
Lines changed: 3 additions & 1 deletion
diff --git a/‎rsocket-core/src/test/java/io/rsocket/metadata/CompositeMetadataFlyweightTest.java
Lines changed: 36 additions & 9 deletions b/‎rsocket-core/src/test/java/io/rsocket/metadata/CompositeMetadataFlyweightTest.java
Lines changed: 36 additions & 9 deletions
@@ -329,13 +329,15 @@ static ByteBuf encodeMetadataHeader(
       ByteBufAllocator allocator, String customMime, int metadataLength) {
     ByteBuf metadataHeader = allocator.buffer(4 + customMime.length());
     // reserve 1 byte for the customMime length
+    // /!\ careful not to read that first byte, which is random at this point
     int writerIndexInitial = metadataHeader.writerIndex();
     metadataHeader.writerIndex(writerIndexInitial + 1);
 
     // write the custom mime in UTF8 but validate it is all ASCII-compatible
     // (which produces the right result since ASCII chars are still encoded on 1 byte in UTF8)
     int customMimeLength = ByteBufUtil.writeUtf8(metadataHeader, customMime);
-    if (!ByteBufUtil.isText(metadataHeader, CharsetUtil.US_ASCII)) {
+    if (!ByteBufUtil.isText(
+        metadataHeader, metadataHeader.readerIndex() + 1, customMimeLength, CharsetUtil.US_ASCII)) {
       metadataHeader.release();
       throw new IllegalArgumentException("custom mime type must be US_ASCII characters only");
     }
 
@@ -19,15 +19,9 @@
 import static io.rsocket.metadata.CompositeMetadataFlyweight.decodeMimeAndContentBuffersSlices;
 import static io.rsocket.metadata.CompositeMetadataFlyweight.decodeMimeIdFromMimeBuffer;
 import static io.rsocket.metadata.CompositeMetadataFlyweight.decodeMimeTypeFromMimeBuffer;
-import static org.assertj.core.api.Assertions.assertThat;
-import static org.assertj.core.api.Assertions.assertThatIllegalArgumentException;
-import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
-
-import io.netty.buffer.ByteBuf;
-import io.netty.buffer.ByteBufAllocator;
-import io.netty.buffer.CompositeByteBuf;
-import io.netty.buffer.Unpooled;
-import io.netty.buffer.UnpooledByteBufAllocator;
+import static org.assertj.core.api.Assertions.*;
+
+import io.netty.buffer.*;
 import io.netty.util.CharsetUtil;
 import io.rsocket.test.util.ByteBufUtils;
 import io.rsocket.util.NumberUtils;
@@ -524,4 +518,37 @@ void knownMimeHeaderZero_avro() {
 
     assertThat(content.readableBytes()).as("no metadata content").isZero();
   }
+
+  @Test
+  void encodeCustomHeaderAsciiCheckSkipsFirstByte() {
+    final ByteBuf badBuf = Unpooled.copiedBuffer("é00000000000", CharsetUtil.UTF_8);
+    badBuf.writerIndex(0);
+    assertThat(badBuf.readerIndex()).isZero();
+
+    ByteBufAllocator allocator =
+        new AbstractByteBufAllocator() {
+          @Override
+          public boolean isDirectBufferPooled() {
+            return false;
+          }
+
+          @Override
+          protected ByteBuf newHeapBuffer(int initialCapacity, int maxCapacity) {
+            return badBuf;
+          }
+
+          @Override
+          protected ByteBuf newDirectBuffer(int initialCapacity, int maxCapacity) {
+            return badBuf;
+          }
+        };
+
+    assertThatCode(
+            () -> CompositeMetadataFlyweight.encodeMetadataHeader(allocator, "custom/type", 0))
+        .doesNotThrowAnyException();
+
+    assertThat(badBuf.readByte()).isEqualTo((byte) 10);
+    assertThat(badBuf.readCharSequence(11, CharsetUtil.UTF_8)).hasToString("custom/type");
+    assertThat(badBuf.readUnsignedMedium()).isEqualTo(0);
+  }
 }