diff --git a/contrib/guix/guix-build b/contrib/guix/guix-build
index 5a908a507d0..dd7229b6fa6 100755
--- a/contrib/guix/guix-build
+++ b/contrib/guix/guix-build
@@ -232,7 +232,7 @@ host_to_commonname() {
 }
 
 # Determine the reference time used for determinism (overridable by environment)
-SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(git log --format=%at -1)}"
+SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(git -c log.showSignature=false log --format=%at -1)}"
 
 # Execute "$@" in a pinned, possibly older version of Guix, for reproducibility
 # across time.
diff --git a/contrib/guix/guix-codesign b/contrib/guix/guix-codesign
index 11610a92e11..3f464f89e61 100755
--- a/contrib/guix/guix-codesign
+++ b/contrib/guix/guix-codesign
@@ -220,7 +220,7 @@ fi
 JOBS="${JOBS:-$(nproc)}"
 
 # Determine the reference time used for determinism (overridable by environment)
-SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(git log --format=%at -1)}"
+SOURCE_DATE_EPOCH="${SOURCE_DATE_EPOCH:-$(git -c log.showSignature=false log --format=%at -1)}"
 
 # Execute "$@" in a pinned, possibly older version of Guix, for reproducibility
 # across time.