diff --git a/iso/builder/build-nccl-tests.sh b/iso/builder/build-nccl-tests.sh
index 4a2f90a..a4352a3 100755
--- a/iso/builder/build-nccl-tests.sh
+++ b/iso/builder/build-nccl-tests.sh
@@ -78,6 +78,11 @@ NCCL_H=$(find . -name 'nccl.h' -type f 2>/dev/null | head -1)
 NCCL_INCLUDE_DIR="$(pwd)/$(dirname "$NCCL_H")"
 echo "nccl.h: $NCCL_H"
 
+# libnccl.so comes from the already-built NCCL cache (build-nccl.sh ran first)
+NCCL_LIB_DIR="${DIST_DIR}/nccl-${NCCL_VERSION}+cuda${NCCL_CUDA_VERSION}/lib"
+[ -d "$NCCL_LIB_DIR" ] || { echo "ERROR: NCCL lib dir not found at $NCCL_LIB_DIR — run build-nccl.sh first"; exit 1; }
+echo "nccl lib: $NCCL_LIB_DIR"
+
 # Download nccl-tests source
 SRC_TAR="${DOWNLOAD_CACHE_DIR}/nccl-tests-v${NCCL_TESTS_VERSION}.tar.gz"
 SRC_URL="https://github.com/NVIDIA/nccl-tests/archive/refs/tags/v${NCCL_TESTS_VERSION}.tar.gz"
@@ -106,6 +111,7 @@ make MPI=0 \
     NVCC="$NVCC" \
     CUDA_HOME="$CUDA_HOME" \
     NCCL_HOME="$NCCL_INCLUDE_DIR/.." \
+    NCCL_LIB="$NCCL_LIB_DIR" \
     NVCC_GENCODE="$GENCODE" \
     BUILDDIR="./build"