diff --git a/NAMESPACE b/NAMESPACE
index 0cb344b..86f31a6 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -110,7 +110,6 @@ importFrom(plyr,ddply)
 importFrom(plyr,desc)
 importFrom(plyr,dlply)
 importFrom(plyr,is.discrete)
-importFrom(plyr,is.formula)
 importFrom(plyr,join)
 importFrom(plyr,ldply)
 importFrom(plyr,llply)
diff --git a/R/ggmap-package.R b/R/ggmap-package.R
index 7b05f38..00a56c1 100644
--- a/R/ggmap-package.R
+++ b/R/ggmap-package.R
@@ -2,7 +2,7 @@
 #' @name ggmap
 #' @importFrom png readPNG
 #' @importFrom jpeg readJPEG
-#' @importFrom plyr arrange compact ddply desc dlply is.discrete is.formula join
+#' @importFrom plyr arrange compact ddply desc dlply is.discrete join
 #'   ldply llply .
 #' @importFrom bitops bitOr bitShiftL bitShiftR bitAnd
 #' @importFrom grDevices as.raster extendrange gray rgb
diff --git a/R/qmplot.R b/R/qmplot.R
index b2af1d4..0a83570 100644
--- a/R/qmplot.R
+++ b/R/qmplot.R
@@ -406,7 +406,7 @@ qmplot <- function(x, y, ..., data, zoom, source = "stadia", maptype = "stamen_t
 
   if (is.null(facets)) {
     p <- p + facet_null()
-  } else if (is.formula(facets) && length(facets) == 2) {
+  } else if (inherits(facets, "formula") && length(facets) == 2) {
     p <- p + facet_wrap(facets)
   } else {
     p <- p + facet_grid(facets = deparse(facets), margins = margins)