diff --git a/Eigen/src/SparseCore/SparseMatrix.h b/Eigen/src/SparseCore/SparseMatrix.h
index b7f437ede..68068124f 100644
--- a/Eigen/src/SparseCore/SparseMatrix.h
+++ b/Eigen/src/SparseCore/SparseMatrix.h
@@ -1550,7 +1550,7 @@ class Serializer<SparseMatrix<Scalar, Options, StorageIndex>, void> {
 
     // innerNonZeros.
     if (!header.compressed) {
-      std:size_t data_bytes = sizeof(StorageIndex) * header.outer_size;
+      std::size_t data_bytes = sizeof(StorageIndex) * header.outer_size;
       memcpy(dest, value.innerNonZeroPtr(), data_bytes);
       dest += data_bytes;
     }