diff --git a/sysbuild.sh b/sysbuild.sh
index 1dd45ff..46ed510 100644
--- a/sysbuild.sh
+++ b/sysbuild.sh
@@ -309,28 +309,53 @@ sysbuild_fetch() {
         cvs)
             local cvsroot="$(shtk_config_get CVSROOT)"
 
+            local srcdir="$(shtk_config_get SRCDIR)"
+            if [ -d "${srcdir}" -a -e "${srcdir}/.git" ]; then
+                shtk_cli_error "SCM=${scm} but ${srcdir} looks" \
+                    "like a Git repo"
+            fi
+
             shtk_cli_info "Updating base source tree"
             shtk_cvs_fetch "${cvsroot}" src "$(shtk_config_get_default CVSTAG '')" \
-                "$(shtk_config_get SRCDIR)"
+                "${srcdir}"
 
             if shtk_config_has XSRCDIR; then
+                local xsrcdir="$(shtk_config_get XSRCDIR)"
+                if [ -d "${xsrcdir}" -a -e "${xsrcdir}/.git" ]; then
+                    shtk_cli_error "SCM=${scm} but ${xsrcdir} looks" \
+                        "like a Git repo"
+                fi
+
                 shtk_cli_info "Updating X11 source tree"
                 shtk_cvs_fetch "${cvsroot}" xsrc \
-                    "$(shtk_config_get_default CVSTAG '')" "$(shtk_config_get XSRCDIR)"
+                    "$(shtk_config_get_default CVSTAG '')" "${xsrcdir}"
             fi
             ;;
 
         git)
             local srcrepo="$(shtk_config_get GIT_SRC_REPO)"
             local srcbranch="$(shtk_config_get GIT_SRC_BRANCH)"
+
+            local srcdir="$(shtk_config_get SRCDIR)"
+            if [ -d "${srcdir}" -a -e "${srcdir}/CVS" ]; then
+                shtk_cli_error "SCM=${scm} but ${srcdir} looks" \
+                    "like a CVS checkout"
+            fi
+
             shtk_cli_info "Updating base source tree"
-            shtk_git_fetch "${srcrepo}" "${srcbranch}" "$(shtk_config_get SRCDIR)"
+            shtk_git_fetch "${srcrepo}" "${srcbranch}" "${srcdir}"
 
             if shtk_config_has XSRCDIR; then
+                local xsrcdir="$(shtk_config_get XSRCDIR)"
+                if [ -d "${xsrcdir}" -a -e "${xsrcdir}/CVS" ]; then
+                    shtk_cli_error "SCM=${scm} but ${xsrcdir} looks" \
+                        "like a CVS checkout"
+                fi
+
                 local xsrcrepo="$(shtk_config_get GIT_XSRC_REPO)"
                 local xsrcbranch="$(shtk_config_get GIT_XSRC_BRANCH)"
                 shtk_cli_info "Updating X11 source tree"
-                shtk_git_fetch "${xsrcrepo}" "${xsrcbranch}" "$(shtk_config_get XSRCDIR)"
+                shtk_git_fetch "${xsrcrepo}" "${xsrcbranch}" "${xsrcdir}"
             fi
             ;;
 
diff --git a/sysbuild_test.sh b/sysbuild_test.sh
index 290f228..151a727 100644
--- a/sysbuild_test.sh
+++ b/sysbuild_test.sh
@@ -1147,6 +1147,78 @@ EOF
 }
 
 
+shtk_unittest_add_test fetch__cvs__update__src_is_git
+fetch__cvs__update__src_is_git_test() {
+    mkdir -p checkout/src/.git
+    cat >test.conf <<EOF
+SCM=cvs
+CVSROOT=irrelevant
+SRCDIR="$(pwd)/checkout/src"
+XSRCDIR=
+EOF
+
+    cat >experr <<EOF
+sysbuild: E: SCM=cvs but $(pwd)/checkout/src looks like a Git repo
+EOF
+    assert_command -s exit:1 -o empty -e file:experr sysbuild -c test.conf fetch
+}
+
+
+shtk_unittest_add_test fetch__cvs__update__xsrc_is_git
+fetch__cvs__update__xsrc_is_git_test() {
+    mock_cvsroot=":local:$(pwd)/cvsroot"
+    create_mock_cvsroot "${mock_cvsroot}"
+    mkdir -p checkout/xsrc/.git
+    cat >test.conf <<EOF
+SCM=cvs
+CVSROOT="${mock_cvsroot}"
+SRCDIR="$(pwd)/checkout/src"
+XSRCDIR="$(pwd)/checkout/xsrc"
+EOF
+
+    assert_command -s exit:1 \
+        -o ignore \
+        -e match:"E: SCM=cvs but $(pwd)/checkout/xsrc looks like a Git repo" \
+        sysbuild -c test.conf fetch
+}
+
+
+shtk_unittest_add_test fetch__git__update__src_is_cvs
+fetch__git__update__src_is_cvs_test() {
+    mkdir -p checkout/src/CVS
+    cat >test.conf <<EOF
+SCM=git
+GIT_SRC_REPO="$(pwd)/src.git"
+SRCDIR="$(pwd)/checkout/src"
+XSRCDIR=
+EOF
+
+    cat >experr <<EOF
+sysbuild: E: SCM=git but $(pwd)/checkout/src looks like a CVS checkout
+EOF
+    assert_command -s exit:1 -o empty -e file:experr sysbuild -c test.conf fetch
+}
+
+
+shtk_unittest_add_test fetch__git__update__xsrc_is_cvs
+fetch__git__update__xsrc_is_cvs_test() {
+    create_mock_git_src "$(pwd)/src.git" trunk
+    mkdir -p checkout/xsrc/CVS
+    cat >test.conf <<EOF
+SCM=git
+GIT_SRC_REPO="$(pwd)/src.git"
+GIT_XSRC_REPO=irrelevant
+SRCDIR="$(pwd)/checkout/src"
+XSRCDIR="$(pwd)/checkout/xsrc"
+EOF
+
+    assert_command -s exit:1 \
+        -o ignore \
+        -e match:"E: SCM=git but $(pwd)/checkout/xsrc looks like a CVS checkout" \
+        sysbuild -c test.conf fetch
+}
+
+
 shtk_unittest_add_test fetch__git__checkout__src_only
 fetch__git__checkout__src_only_test() {
     create_mock_git_src "$(pwd)/src.git" trunk