diff --git a/pkgs/applications/misc/opencpn/default.nix b/pkgs/applications/misc/opencpn/default.nix
index 984747585476..a39d5aca3f3f 100644
--- a/pkgs/applications/misc/opencpn/default.nix
+++ b/pkgs/applications/misc/opencpn/default.nix
@@ -58,6 +58,11 @@ stdenv.mkDerivation (finalAttrs: {
     hash = "sha256-1JCb2aYyjaiUvtYkBFtEdlClmiMABN3a/Hts9V1sbgc=";
   };
 
+  patches = [
+    # https://github.com/OpenCPN/OpenCPN/pull/4900
+    ./fix-clang20.patch
+  ];
+
   postPatch = lib.optionalString stdenv.hostPlatform.isDarwin ''
     sed -i '/fixup_bundle/d; /NO_DEFAULT_PATH/d' CMakeLists.txt
   '';
diff --git a/pkgs/applications/misc/opencpn/fix-clang20.patch b/pkgs/applications/misc/opencpn/fix-clang20.patch
new file mode 100644
index 000000000000..cfc5e5c3f9f8
--- /dev/null
+++ b/pkgs/applications/misc/opencpn/fix-clang20.patch
@@ -0,0 +1,82 @@
+From cb5d28c48c814454182a5169e852f2596ba018ae Mon Sep 17 00:00:00 2001
+From: Ihar Hrachyshka <ihar.hrachyshka@gmail.com>
+Date: Thu, 13 Nov 2025 22:25:56 -0500
+Subject: [PATCH] Fix clang 20 build failure (-Wnontrivial-memcall), attempt 2
+
+The previous attempt [1] resulted in -Werror=use-after-free violation
+when built with gcc on bookworm because `*this` triggered the class
+destructor, calling `free` on an uninitialized pointer.
+
+In this new patch, we are going to use an explicit initializer list to
+initialize all class members with zero values.
+
+[1] d3eb15de1e5aa8f85890d5c83cbc025ac7a467be
+---
+ gui/src/IDX_entry.cpp | 52 ++++++++++++++++++++++++++++++++++++++++++-
+ 1 file changed, 51 insertions(+), 1 deletion(-)
+
+diff --git a/gui/src/IDX_entry.cpp b/gui/src/IDX_entry.cpp
+index 726833ccc..46923d633 100644
+--- a/gui/src/IDX_entry.cpp
++++ b/gui/src/IDX_entry.cpp
+@@ -29,6 +29,56 @@
+ 
+ WX_DEFINE_OBJARRAY(ArrayOfIDXEntry);
+ 
+-IDX_entry::IDX_entry() { memset(this, 0, sizeof(IDX_entry)); }
++IDX_entry::IDX_entry()
++    : source_data_type(SOURCE_TYPE_UNKNOWN),
++      pDataSource(nullptr),
++      IDX_rec_num(0),
++      IDX_type(0),
++      IDX_lon(0.0),
++      IDX_lat(0.0),
++      IDX_ht_time_off(0),
++      IDX_ht_mpy(0.0f),
++      IDX_ht_off(0.0f),
++      IDX_lt_time_off(0),
++      IDX_lt_mpy(0.0f),
++      IDX_lt_off(0.0f),
++      IDX_sta_num(0),
++      IDX_flood_dir(0),
++      IDX_ebb_dir(0),
++      IDX_Useable(0),
++      Valid15(0),
++      Value15(0.0f),
++      Dir15(0.0f),
++      Ret15(false),
++      IDX_tzname(nullptr),
++      IDX_ref_file_num(0),
++      IDX_ref_dbIndex(0),
++      max_amplitude(0.0),
++      have_offsets(0),
++      station_tz_offset(0),
++      IDX_time_zone(0),
++      pref_sta_data(nullptr),
++      num_nodes(0),
++      num_csts(0),
++      num_epochs(0),
++      m_cst_speeds(nullptr),
++      m_cst_nodes(nullptr),
++      m_cst_epochs(nullptr),
++      m_work_buffer(nullptr),
++      first_year(0),
++      epoch(0),
++      epoch_year(0),
++      current_depth(0),
++      b_skipTooDeep(false),
++      recent_highlow_calc_time(0),
++      recent_high_level(0.0f),
++      recent_high_time(0),
++      recent_low_level(0.0f),
++      recent_low_time(0) {
++  memset(source_ident, 0, sizeof(source_ident));
++  memset(IDX_zone, 0, sizeof(IDX_zone));
++  memset(IDX_station_name, 0, sizeof(IDX_station_name));
++  memset(IDX_reference_name, 0, sizeof(IDX_reference_name));
++}
+ 
+ IDX_entry::~IDX_entry() { free(IDX_tzname); }
+-- 
+2.51.0
+