diff --git a/R/readNWISdata.r b/R/readNWISdata.r
index 28d2907ef28dfde27c255b6f5ebe190ffdb7e1e7..8b54a4cffca92f2b1a0fb21a7360adc8db4dc08d 100644
--- a/R/readNWISdata.r
+++ b/R/readNWISdata.r
@@ -50,6 +50,10 @@ readNWISdata <- function(service="dv", ...){
   
   matchReturn <- list(...)
   
+  if(length(service) > 1){
+    stop("Only one service call allowed.")
+  }
+  
   values <- sapply(matchReturn, function(x) URLencode(as.character(paste(eval(x),collapse=",",sep=""))))
   
   urlCall <- paste(paste(names(values),values,sep="="),collapse="&")