diff --git a/era/era_invalidate.cc b/era/era_invalidate.cc
index c81b5af..20490a0 100644
--- a/era/era_invalidate.cc
+++ b/era/era_invalidate.cc
@@ -152,7 +152,7 @@ namespace {
 	int invalidate(string const &dev, string const &output, flags const &fs) {
 		try {
 			set<uint32_t> blocks;
-			block_manager<>::ptr bm = open_bm(dev, block_manager<>::READ_ONLY);
+			block_manager<>::ptr bm = open_bm(dev, block_manager<>::READ_ONLY, !fs.metadata_snapshot_);
 
 			if (fs.metadata_snapshot_) {
 				superblock sb = read_superblock(bm);
diff --git a/man8/era_invalidate.8 b/man8/era_invalidate.8
index 2e67d4a..9bc6994 100644
--- a/man8/era_invalidate.8
+++ b/man8/era_invalidate.8
@@ -11,7 +11,7 @@ era_invalidate \- Provide a list of blocks that have changed since a particular
 .B era_invalidate
 Examines era metadata and lists blocks that may have changed since a given era.
 
-This tool cannot be run on live metadata.
+This tool cannot be run on live metadata unless the \fB\-\-metadata\-snap\fP option is used.
 
 .SH OPTIONS
 .IP "\fB\-h, \-\-help\fP"