diff --git a/httpcore5/src/main/java/org/apache/hc/core5/http/impl/nio/SessionOutputBufferImpl.java b/httpcore5/src/main/java/org/apache/hc/core5/http/impl/nio/SessionOutputBufferImpl.java
index 77cd9951fa..0ef5611fda 100644
--- a/httpcore5/src/main/java/org/apache/hc/core5/http/impl/nio/SessionOutputBufferImpl.java
+++ b/httpcore5/src/main/java/org/apache/hc/core5/http/impl/nio/SessionOutputBufferImpl.java
@@ -41,6 +41,7 @@
 import org.apache.hc.core5.http.nio.SessionOutputBuffer;
 import org.apache.hc.core5.util.Args;
 import org.apache.hc.core5.util.CharArrayBuffer;
+import org.apache.hc.core5.util.TextUtils;
 
 class SessionOutputBufferImpl extends ExpandableBuffer implements SessionOutputBuffer {
 
@@ -171,12 +172,14 @@ public void writeLine(final CharArrayBuffer lineBuffer) throws CharacterCodingEx
                     final int off = buffer().position();
                     final int arrayOffset = buffer().arrayOffset();
                     for (int i = 0; i < len; i++) {
-                        b[arrayOffset + off + i]  = (byte) lineBuffer.charAt(i);
+                        final int c = lineBuffer.charAt(i);
+                        b[arrayOffset + off + i] = TextUtils.castAsByte(c);
                     }
                     buffer().position(off + len);
                 } else {
                     for (int i = 0; i < lineBuffer.length(); i++) {
-                        buffer().put((byte) lineBuffer.charAt(i));
+                        final int c = lineBuffer.charAt(i);
+                        buffer().put(TextUtils.castAsByte(c));
                     }
                 }
             } else {
diff --git a/httpcore5/src/main/java/org/apache/hc/core5/util/ByteArrayBuffer.java b/httpcore5/src/main/java/org/apache/hc/core5/util/ByteArrayBuffer.java
index c9479c320e..819ad7d3f4 100644
--- a/httpcore5/src/main/java/org/apache/hc/core5/util/ByteArrayBuffer.java
+++ b/httpcore5/src/main/java/org/apache/hc/core5/util/ByteArrayBuffer.java
@@ -138,14 +138,7 @@ public void append(final char[] b, final int off, final int len) {
         }
 
         for (int i1 = off, i2 = oldlen; i2 < newlen; i1++, i2++) {
-            final int c = b[i1];
-            if ((c >= 0x20 && c <= 0x7E) || // Visible ASCII
-                (c >= 0xA0 && c <= 0xFF) || // Visible ISO-8859-1
-                c == 0x09) {                // TAB
-                this.array[i2] = (byte) c;
-            } else {
-                this.array[i2] = '?';
-            }
+            this.array[i2] = TextUtils.castAsByte(b[i1]);
         }
         this.len = newlen;
     }
diff --git a/httpcore5/src/main/java/org/apache/hc/core5/util/TextUtils.java b/httpcore5/src/main/java/org/apache/hc/core5/util/TextUtils.java
index a8b6a6e280..db4c948bc9 100644
--- a/httpcore5/src/main/java/org/apache/hc/core5/util/TextUtils.java
+++ b/httpcore5/src/main/java/org/apache/hc/core5/util/TextUtils.java
@@ -29,6 +29,8 @@
 
 import java.util.Locale;
 
+import org.apache.hc.core5.annotation.Internal;
+
 /**
  * @since 4.3
  */
@@ -141,4 +143,21 @@ public static String toLowerCase(final String s) {
         return s.toLowerCase(Locale.ROOT);
     }
 
+    /**
+     * Casts character to byte filtering non-visible and non-ASCII characters
+     * before conversion
+     *
+     * @since 5.2
+     */
+    @Internal
+    public static byte castAsByte(final int c) {
+        if ((c >= 0x20 && c <= 0x7E) || // Visible ASCII
+            (c >= 0xA0 && c <= 0xFF) || // Visible ISO-8859-1
+             c == 0x09) {               // TAB
+            return (byte) c;
+        } else {
+            return '?';
+        }
+    }
+
 }
diff --git a/httpcore5/src/test/java/org/apache/hc/core5/http/impl/nio/TestSessionInOutBuffers.java b/httpcore5/src/test/java/org/apache/hc/core5/http/impl/nio/TestSessionInOutBuffers.java
index 7579a1fc53..0f4d1bba89 100644
--- a/httpcore5/src/test/java/org/apache/hc/core5/http/impl/nio/TestSessionInOutBuffers.java
+++ b/httpcore5/src/test/java/org/apache/hc/core5/http/impl/nio/TestSessionInOutBuffers.java
@@ -194,6 +194,34 @@ public void testWriteLineChunks() throws Exception {
         Assertions.assertEquals("One\r\nTwo\r\nThree\r\nFour\r\n", s);
     }
 
+    @Test
+    public void testNonASCIIWriteLine() throws Exception {
+        final String testString = "123\u010Anew-header-from-some-header:injected-value";
+        final String expectedResult = "123?new-header-from-some-header:injected-value";
+
+        final CharArrayBuffer chbuffer = new CharArrayBuffer(32);
+        final SessionOutputBuffer outbuf = new SessionOutputBufferImpl(1024, 16);
+
+        chbuffer.clear();
+        chbuffer.append(testString);
+        outbuf.writeLine(chbuffer);
+
+        //this write operation should have no effect
+        outbuf.writeLine(null);
+
+        final ByteArrayOutputStream outStream = new ByteArrayOutputStream();
+        final WritableByteChannel outChannel = newChannel(outStream);
+        outbuf.flush(outChannel);
+
+        final ReadableByteChannel channel = newChannel(outStream.toByteArray());
+
+        final SessionInputBuffer inbuf = new SessionInputBufferImpl(1024, 16, 0);
+        inbuf.fill(channel);
+        chbuffer.clear();
+        inbuf.readLine(chbuffer, true);
+        Assertions.assertEquals(expectedResult, chbuffer.toString());
+    }
+
     @Test
     public void testBasicReadWriteLine() throws Exception {