diff --git a/ChangeLog.md b/ChangeLog.md
index ed410210..643d9e64 100644
--- a/ChangeLog.md
+++ b/ChangeLog.md
@@ -19,6 +19,11 @@ was previously enabled in a libjpeg or TurboJPEG instance.
 instance, and setting `TJPARAM_LOSSLESS`/`TJ.PARAM_LOSSLESS` to `0` now
 disables lossless JPEG compression in a TurboJPEG instance.
 
+4. Hardened the libjpeg API against hypothetical calling applications that may
+erroneously change the value of the `data_precision` field in
+`jpeg_compress_struct` or `jpeg_decompress_struct` after calling
+`jpeg_start_compress()` or `jpeg_start_decompress()`.
+
 
 3.0.4
 =====
diff --git a/jcapimin.c b/jcapimin.c
index cbb3d13e..7ddb09b7 100644
--- a/jcapimin.c
+++ b/jcapimin.c
@@ -5,7 +5,7 @@
  * Copyright (C) 1994-1998, Thomas G. Lane.
  * Modified 2003-2010 by Guido Vollbeding.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2022, D. R. Commander.
+ * Copyright (C) 2022, 2024, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -196,15 +196,21 @@ jpeg_finish_compress(j_compress_ptr cinfo)
        */
       if (cinfo->data_precision == 16) {
 #ifdef C_LOSSLESS_SUPPORTED
+        if (cinfo->coef->compress_data_16 == NULL)
+          ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
         if (!(*cinfo->coef->compress_data_16) (cinfo, (J16SAMPIMAGE)NULL))
           ERREXIT(cinfo, JERR_CANT_SUSPEND);
 #else
         ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
 #endif
       } else if (cinfo->data_precision == 12) {
+        if (cinfo->coef->compress_data_12 == NULL)
+          ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
         if (!(*cinfo->coef->compress_data_12) (cinfo, (J12SAMPIMAGE)NULL))
           ERREXIT(cinfo, JERR_CANT_SUSPEND);
       } else {
+        if (cinfo->coef->compress_data == NULL)
+          ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
         if (!(*cinfo->coef->compress_data) (cinfo, (JSAMPIMAGE)NULL))
           ERREXIT(cinfo, JERR_CANT_SUSPEND);
       }
diff --git a/jcapistd.c b/jcapistd.c
index 2053028f..220c728d 100644
--- a/jcapistd.c
+++ b/jcapistd.c
@@ -4,7 +4,7 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1994-1996, Thomas G. Lane.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2022, D. R. Commander.
+ * Copyright (C) 2022, 2024, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -117,6 +117,8 @@ _jpeg_write_scanlines(j_compress_ptr cinfo, _JSAMPARRAY scanlines,
     num_lines = rows_left;
 
   row_ctr = 0;
+  if (cinfo->main->_process_data == NULL)
+    ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
   (*cinfo->main->_process_data) (cinfo, scanlines, &row_ctr, num_lines);
   cinfo->next_scanline += row_ctr;
   return row_ctr;
@@ -174,6 +176,8 @@ _jpeg_write_raw_data(j_compress_ptr cinfo, _JSAMPIMAGE data,
     ERREXIT(cinfo, JERR_BUFFER_SIZE);
 
   /* Directly compress the row. */
+  if (cinfo->coef->_compress_data == NULL)
+    ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
   if (!(*cinfo->coef->_compress_data) (cinfo, data)) {
     /* If compressor did not consume the whole row, suspend processing. */
     return 0;
diff --git a/jccoefct.c b/jccoefct.c
index 2a5dde2d..4a47b3cb 100644
--- a/jccoefct.c
+++ b/jccoefct.c
@@ -4,7 +4,7 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1994-1997, Thomas G. Lane.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2022, D. R. Commander.
+ * Copyright (C) 2022, 2024, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -414,6 +414,7 @@ _jinit_c_coef_controller(j_compress_ptr cinfo, boolean need_full_buffer)
   coef = (my_coef_ptr)
     (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                                 sizeof(my_coef_controller));
+  memset(coef, 0, sizeof(my_coef_controller));
   cinfo->coef = (struct jpeg_c_coef_controller *)coef;
   coef->pub.start_pass = start_pass_coef;
 
diff --git a/jcmainct.c b/jcmainct.c
index fe8fc0b1..3db2e649 100644
--- a/jcmainct.c
+++ b/jcmainct.c
@@ -6,7 +6,7 @@
  * Lossless JPEG Modifications:
  * Copyright (C) 1999, Ken Murchison.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2022, D. R. Commander.
+ * Copyright (C) 2022, 2024, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -146,6 +146,7 @@ _jinit_c_main_controller(j_compress_ptr cinfo, boolean need_full_buffer)
   main_ptr = (my_main_ptr)
     (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                                 sizeof(my_main_controller));
+  memset(main_ptr, 0, sizeof(my_main_controller));
   cinfo->main = (struct jpeg_c_main_controller *)main_ptr;
   main_ptr->pub.start_pass = start_pass_main;
 
diff --git a/jdapistd.c b/jdapistd.c
index 38da36b7..3a3aa602 100644
--- a/jdapistd.c
+++ b/jdapistd.c
@@ -128,20 +128,28 @@ output_pass_setup(j_decompress_ptr cinfo)
       }
       /* Process some data */
       last_scanline = cinfo->output_scanline;
+      if (cinfo->data_precision == 16) {
 #ifdef D_LOSSLESS_SUPPORTED
-      if (cinfo->data_precision == 16)
+        if (cinfo->main->process_data_16 == NULL)
+          ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
         (*cinfo->main->process_data_16) (cinfo, (J16SAMPARRAY)NULL,
                                          &cinfo->output_scanline,
                                          (JDIMENSION)0);
-      else
+#else
+        ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
 #endif
-      if (cinfo->data_precision == 12)
+      } else if (cinfo->data_precision == 12) {
+        if (cinfo->main->process_data_12 == NULL)
+          ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
         (*cinfo->main->process_data_12) (cinfo, (J12SAMPARRAY)NULL,
                                          &cinfo->output_scanline,
                                          (JDIMENSION)0);
-      else
+      } else {
+        if (cinfo->main->process_data == NULL)
+          ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
         (*cinfo->main->process_data) (cinfo, (JSAMPARRAY)NULL,
                                       &cinfo->output_scanline, (JDIMENSION)0);
+      }
       if (cinfo->output_scanline == last_scanline)
         return FALSE;           /* No progress made, must suspend */
     }
@@ -333,6 +341,8 @@ _jpeg_read_scanlines(j_decompress_ptr cinfo, _JSAMPARRAY scanlines,
 
   /* Process some data */
   row_ctr = 0;
+  if (cinfo->main->_process_data == NULL)
+    ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
   (*cinfo->main->_process_data) (cinfo, scanlines, &row_ctr, max_lines);
   cinfo->output_scanline += row_ctr;
   return row_ctr;
@@ -679,6 +689,8 @@ _jpeg_read_raw_data(j_decompress_ptr cinfo, _JSAMPIMAGE data,
     ERREXIT(cinfo, JERR_BUFFER_SIZE);
 
   /* Decompress directly into user's buffer. */
+  if (cinfo->coef->_decompress_data == NULL)
+    ERREXIT1(cinfo, JERR_BAD_PRECISION, cinfo->data_precision);
   if (!(*cinfo->coef->_decompress_data) (cinfo, data))
     return 0;                   /* suspension forced, can do nothing more */
 
diff --git a/jdcoefct.c b/jdcoefct.c
index 40ce2725..194d5a5c 100644
--- a/jdcoefct.c
+++ b/jdcoefct.c
@@ -5,7 +5,7 @@
  * Copyright (C) 1994-1997, Thomas G. Lane.
  * libjpeg-turbo Modifications:
  * Copyright 2009 Pierre Ossman <ossman@cendio.se> for Cendio AB
- * Copyright (C) 2010, 2015-2016, 2019-2020, 2022-2023, D. R. Commander.
+ * Copyright (C) 2010, 2015-2016, 2019-2020, 2022-2024, D. R. Commander.
  * Copyright (C) 2015, 2020, Google, Inc.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
@@ -824,6 +824,7 @@ _jinit_d_coef_controller(j_decompress_ptr cinfo, boolean need_full_buffer)
   coef = (my_coef_ptr)
     (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                                 sizeof(my_coef_controller));
+  memset(coef, 0, sizeof(my_coef_controller));
   cinfo->coef = (struct jpeg_d_coef_controller *)coef;
   coef->pub.start_input_pass = start_input_pass;
   coef->pub.start_output_pass = start_output_pass;
diff --git a/jdmainct.c b/jdmainct.c
index c672b4ba..4ed7a927 100644
--- a/jdmainct.c
+++ b/jdmainct.c
@@ -4,7 +4,7 @@
  * This file was part of the Independent JPEG Group's software:
  * Copyright (C) 1994-1996, Thomas G. Lane.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2010, 2016, 2022, D. R. Commander.
+ * Copyright (C) 2010, 2016, 2022, 2024, D. R. Commander.
  * For conditions of distribution and use, see the accompanying README.ijg
  * file.
  *
@@ -437,6 +437,7 @@ _jinit_d_main_controller(j_decompress_ptr cinfo, boolean need_full_buffer)
   main_ptr = (my_main_ptr)
     (*cinfo->mem->alloc_small) ((j_common_ptr)cinfo, JPOOL_IMAGE,
                                 sizeof(my_main_controller));
+  memset(main_ptr, 0, sizeof(my_main_controller));
   cinfo->main = (struct jpeg_d_main_controller *)main_ptr;
   main_ptr->pub.start_pass = start_pass_main;