diff --git a/iso/builder/build-nvidia-module.sh b/iso/builder/build-nvidia-module.sh
index c54f77c..8c06f9a 100644
--- a/iso/builder/build-nvidia-module.sh
+++ b/iso/builder/build-nvidia-module.sh
@@ -23,17 +23,24 @@ DEBIAN_KERNEL_ABI="$3"
 [ -n "$DEBIAN_KERNEL_ABI" ] || { echo "usage: $0 <nvidia-version> <dist-dir> <debian-kernel-abi>"; exit 1; }
 
 KVER="${DEBIAN_KERNEL_ABI}-amd64"
-KDIR="/usr/src/linux-headers-${KVER}"
+# On Debian, kernel headers are split into two packages:
+#   linux-headers-<kver>        — arch-specific (generated, Makefile)
+#   linux-headers-<kver>-common — common source headers (linux/, asm-generic/, etc.)
+# NVIDIA conftest needs SYSSRC pointing to common (for source headers like linux/mm.h)
+# and SYSOUT pointing to amd64 (for generated headers like autoconf.h, asm/).
+KDIR_ARCH="/usr/src/linux-headers-${KVER}"
+KDIR_COMMON="/usr/src/linux-headers-${DEBIAN_KERNEL_ABI}-common"
 
 echo "=== NVIDIA ${NVIDIA_VERSION} (proprietary) for kernel ${KVER} ==="
 
-if [ ! -d "$KDIR" ]; then
+if [ ! -d "$KDIR_ARCH" ] || [ ! -d "$KDIR_COMMON" ]; then
     echo "=== installing linux-headers-${KVER} ==="
     DEBIAN_FRONTEND=noninteractive apt-get install -y \
         "linux-headers-${KVER}" \
         gcc make perl
 fi
-echo "kernel headers: $KDIR"
+echo "kernel headers (arch):   $KDIR_ARCH"
+echo "kernel headers (common): $KDIR_COMMON"
 
 CACHE_DIR="${DIST_DIR}/nvidia-${NVIDIA_VERSION}-${KVER}"
 if [ -d "$CACHE_DIR/modules" ] && [ -f "$CACHE_DIR/bin/nvidia-smi" ]; then
@@ -86,12 +93,16 @@ echo "kernel source: $KERNEL_SRC"
 # Build kernel modules
 # CFLAGS_MODULE: add GCC include dir so NVIDIA's nv_stdarg.h can find stdarg.h.
 # Kernel build uses -nostdinc which strips GCC's own includes; we restore it here.
-GCC_INCLUDES=$(gcc --print-file-name=include 2>/dev/null || echo "")
 echo "=== building kernel modules ($(nproc) cores) ==="
 cd "$KERNEL_SRC"
+# SYSSRC=common: conftest finds real kernel headers (linux/mm.h etc.)
+# SYSOUT=amd64:  generated headers (autoconf.h, asm/) from arch package
+# Without this split, conftest uses amd64/include/ which is nearly empty,
+# all compile-tests fail silently, and NVIDIA assumes all APIs present → link errors.
 make -j$(nproc) \
-    KERNEL_UNAME="$KVER" SYSSRC="$KDIR" \
-    CFLAGS_MODULE="${GCC_INCLUDES:+-I${GCC_INCLUDES}} -Wno-error" \
+    KERNEL_UNAME="$KVER" \
+    SYSSRC="$KDIR_COMMON" \
+    SYSOUT="$KDIR_ARCH" \
     modules 2>&1 | tail -10
 
 # Collect outputs