diff --git a/common/GitInfoStamp.cmake b/common/GitInfoStamp.cmake
index f2c9ce138..148993a98 100644
--- a/common/GitInfoStamp.cmake
+++ b/common/GitInfoStamp.cmake
@@ -4,7 +4,22 @@
 # reported a hash seventeen commits stale. configure_file leaves the output untouched when nothing
 # changed, so this costs a recompile only when HEAD moves. Also included at configure time, so the
 # file exists for the generator and the SHA prints with the configure output.
+# A source tree that is not a git checkout of its own - a downloaded tarball, perhaps unpacked inside
+# some other repository - has no commit to report: the version from VERSION stands alone and the
+# commit reads "unknown", and the build goes on.
+SET(GIT_SHA1 "")
+SET(GIT_DATE "")
 if (GIT_EXECUTABLE)
+    EXECUTE_PROCESS(COMMAND "${GIT_EXECUTABLE}" rev-parse --show-toplevel
+            WORKING_DIRECTORY "${GITINFO_SOURCE_DIR}"
+            OUTPUT_VARIABLE GIT_TOPLEVEL
+            ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)
+    if (GIT_TOPLEVEL)
+        FILE(REAL_PATH "${GIT_TOPLEVEL}" GIT_TOPLEVEL)
+    endif ()
+    FILE(REAL_PATH "${GITINFO_SOURCE_DIR}" GITINFO_SOURCE_REAL)
+endif ()
+if (GIT_EXECUTABLE AND GIT_TOPLEVEL STREQUAL GITINFO_SOURCE_REAL)
     # --dirty: a build from uncommitted changes says so in the hash itself
     EXECUTE_PROCESS(COMMAND
             "${GIT_EXECUTABLE}" describe --match=NeVeRmAtCh --always --abbrev=8 --dirty
@@ -17,4 +32,7 @@ if (GIT_EXECUTABLE)
             OUTPUT_VARIABLE GIT_DATE
             ERROR_QUIET OUTPUT_STRIP_TRAILING_WHITESPACE)
 endif ()
+if (NOT GIT_SHA1)
+    SET(GIT_SHA1 "unknown")
+endif ()
 CONFIGURE_FILE("${GITINFO_TEMPLATE}" "${GITINFO_OUTPUT}" @ONLY)
diff --git a/image_analysis/WriteReflections.cpp b/image_analysis/WriteReflections.cpp
index 7b49f877a..fba15332c 100644
--- a/image_analysis/WriteReflections.cpp
+++ b/image_analysis/WriteReflections.cpp
@@ -110,9 +110,10 @@ std::string IsoDate(const std::string &timestamp) {
     return timestamp.substr(0, 10);
 }
 
-// The commit this build was made from, with its date; empty when the build does not know it.
+// The commit this build was made from, with its date where the build knows it ("unknown" for a source
+// tree without git history).
 std::string GitDescription() {
-    return jfjoch_git_sha1().empty() ? std::string() : jfjoch_git_sha1() + " " + jfjoch_git_date();
+    return jfjoch_git_date().empty() ? jfjoch_git_sha1() : jfjoch_git_sha1() + " " + jfjoch_git_date();
 }
 
 // The MTZ HISTORY records saying where a file came from, after the one naming the program: the
@@ -284,7 +285,7 @@ void WriteMmcifReflections(const std::vector<MergedReflection> &reflections,
     // Rugnux integrated the images (data reduction) and scaled and merged them (data scaling), the
     // two rows the wwPDB asks a data-processing program to be named under; the description is the
     // commit the build was made from.
-    const std::string software_description = CifStr(GitDescription().empty() ? "" : "git " + GitDescription());
+    const std::string software_description = CifStr("git " + GitDescription());
     out << "loop_\n";
     out << "_software.pdbx_ordinal\n";
     out << "_software.name\n";