diff --git a/ChangeLog.d/clean_pem_buffers.txt b/ChangeLog.d/clean_pem_buffers.txt
new file mode 100644
index 0000000..818fad9
--- /dev/null
+++ b/ChangeLog.d/clean_pem_buffers.txt
@@ -0,0 +1,6 @@
+Bugfix
+  * In PEM writing functions, fill the trailing part of the buffer with null
+    bytes. This guarantees that the corresponding parsing function can read
+    the buffer back, which was the case for mbedtls_x509write_{crt,csr}_pem
+    until this property was inadvertently broken in Mbed TLS 2.19.0.
+    Fixes #3682.
diff --git a/library/pem.c b/library/pem.c
index 534d071..969d492 100644
--- a/library/pem.c
+++ b/library/pem.c
@@ -478,8 +478,12 @@
     *p++ = '\0';
     *olen = p - buf;
 
+     /* Clean any remaining data previously written to the buffer */
+    memset( buf + *olen, 0, buf_len - *olen );
+
     mbedtls_free( encode_buf );
     return( 0 );
 }
 #endif /* MBEDTLS_PEM_WRITE_C */
 #endif /* MBEDTLS_PEM_PARSE_C || MBEDTLS_PEM_WRITE_C */
+
diff --git a/tests/suites/test_suite_pkwrite.function b/tests/suites/test_suite_pkwrite.function
index 43c275e..2bad4ed 100644
--- a/tests/suites/test_suite_pkwrite.function
+++ b/tests/suites/test_suite_pkwrite.function
@@ -17,7 +17,7 @@
     unsigned char check_buf[5000];
     int ret;
     FILE *f;
-    size_t ilen;
+    size_t ilen, pem_len, buf_index;
 
     memset( buf, 0, sizeof( buf ) );
     memset( check_buf, 0, sizeof( check_buf ) );
@@ -28,12 +28,20 @@
     ret = mbedtls_pk_write_pubkey_pem( &key, buf, sizeof( buf ));
     TEST_ASSERT( ret == 0 );
 
+    pem_len = strlen( (char *) buf );
+
+    // check that the rest of the buffer remains clear
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( key_file, "r" );
     TEST_ASSERT( f != NULL );
     ilen = fread( check_buf, 1, sizeof( check_buf ), f );
     fclose( f );
 
-    TEST_ASSERT( ilen == strlen( (char *) buf ) );
+    TEST_ASSERT( ilen == pem_len );
     TEST_ASSERT( memcmp( (char *) buf, (char *) check_buf, ilen ) == 0 );
 
 exit:
@@ -49,7 +57,7 @@
     unsigned char check_buf[5000];
     int ret;
     FILE *f;
-    size_t ilen;
+    size_t ilen, pem_len, buf_index;
 
     memset( buf, 0, sizeof( buf ) );
     memset( check_buf, 0, sizeof( check_buf ) );
@@ -60,6 +68,14 @@
     ret = mbedtls_pk_write_key_pem( &key, buf, sizeof( buf ));
     TEST_ASSERT( ret == 0 );
 
+    pem_len = strlen( (char *) buf );
+
+    // check that the rest of the buffer remains clear
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( key_file, "r" );
     TEST_ASSERT( f != NULL );
     ilen = fread( check_buf, 1, sizeof( check_buf ), f );
diff --git a/tests/suites/test_suite_x509write.function b/tests/suites/test_suite_x509write.function
index 9f2007d..3803377 100644
--- a/tests/suites/test_suite_x509write.function
+++ b/tests/suites/test_suite_x509write.function
@@ -104,7 +104,7 @@
     unsigned char buf[4096];
     unsigned char check_buf[4000];
     int ret;
-    size_t olen = 0, pem_len = 0;
+    size_t olen = 0, pem_len = 0, buf_index;
     int der_len = -1;
     FILE *f;
     const char *subject_name = "C=NL,O=PolarSSL,CN=PolarSSL Server 1";
@@ -130,6 +130,11 @@
 
     pem_len = strlen( (char *) buf );
 
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( cert_req_check_file, "r" );
     TEST_ASSERT( f != NULL );
     olen = fread( check_buf, 1, sizeof( check_buf ), f );
@@ -224,7 +229,7 @@
     unsigned char check_buf[5000];
     mbedtls_mpi serial;
     int ret;
-    size_t olen = 0, pem_len = 0;
+    size_t olen = 0, pem_len = 0, buf_index = 0;
     int der_len = -1;
     FILE *f;
     mbedtls_test_rnd_pseudo_info rnd_info;
@@ -293,6 +298,12 @@
 
     pem_len = strlen( (char *) buf );
 
+    // check that the rest of the buffer remains clear
+    for( buf_index = pem_len; buf_index < sizeof( buf ); ++buf_index )
+    {
+        TEST_ASSERT( buf[buf_index] == 0 );
+    }
+
     f = fopen( cert_check_file, "r" );
     TEST_ASSERT( f != NULL );
     olen = fread( check_buf, 1, sizeof( check_buf ), f );