diff --git a/R/sgd_lm.R b/R/sgd_lm.R
index db295c9..b2c77a3 100644
--- a/R/sgd_lm.R
+++ b/R/sgd_lm.R
@@ -51,7 +51,7 @@ sgd_lm = function(formula,
   mf <- eval(mf, parent.frame())
   mt <- attr(mf, "terms")
   y <- model.response(mf, "numeric")
-  x <- model.matrix(mt, mf)[,-1]
+  x <- as.matrix(model.matrix(mt, mf, drop=F)[,-1])
   
   if (studentize){
     # Compute column means and standard errors and save them for later reconversion
diff --git a/R/sgd_qr.R b/R/sgd_qr.R
index ac517e4..35bc8fa 100644
--- a/R/sgd_qr.R
+++ b/R/sgd_qr.R
@@ -55,7 +55,7 @@ sgd_qr = function(formula,
   mf <- eval(mf, parent.frame())
   mt <- attr(mf, "terms")
   y <- model.response(mf, "numeric")
-  x <- model.matrix(mt, mf)[,-1]
+  x <- as.matrix(model.matrix(mt, mf, drop=F)[,-1])
   
   if (studentize){
     # Compute column means and standard errors and save them for later reconversion
diff --git a/R/sgdi_qr.R b/R/sgdi_qr.R
index f1e201c..3bcfcf5 100644
--- a/R/sgdi_qr.R
+++ b/R/sgdi_qr.R
@@ -74,7 +74,7 @@ sgdi_qr = function(formula,
   mf <- eval(mf, parent.frame())
   mt <- attr(mf, "terms")
   y <- model.response(mf, "numeric")
-  x <- model.matrix(mt, mf)[,-1]
+  x <- as.matrix(model.matrix(mt, mf, drop=F)[,-1])
   if (inference == "rss"){
     if (0 %in% rss_idx ){
       stop("rss_idx includes 0 (the intercept term), where it should be bigger than 1.")