diff --git a/R/InfinitySparseMatrix.R b/R/InfinitySparseMatrix.R
index e6c674ca..fa7e498b 100644
--- a/R/InfinitySparseMatrix.R
+++ b/R/InfinitySparseMatrix.R
@@ -1120,6 +1120,12 @@ as.list.DenseMatrix <- function(x, ...) {
 subset.BlockedInfinitySparseMatrix <- function(x, subset, select, ...) {
     subIsm <- callGeneric(as(x, "InfinitySparseMatrix"), subset, select)
     oldNames <- names(x@groups)
+    if (!all(union(x@rownames, x@colnames) %in% oldNames)) {
+        warning(paste("Subsetting BlockedInfinitySparseMatrix with",
+                      "rownames union colnames not set equal to",
+                      "group names results in an InfinitySparseMatrix."))
+        return(subIsm)
+    }
     if (!is.null(oldNames)) {   # we can use the groups names to subset groups
         subNames <- oldNames[which((oldNames %in% subIsm@rownames) |
                                    (oldNames %in% subIsm@colnames))]