diff --git a/NEWS b/NEWS
index 978049384b9fa0a30df93f731d4275a7e94f1c39..4f136e18489c6ce198e87da787fb03072c8f2bd4 100755
--- a/NEWS
+++ b/NEWS
@@ -3,9 +3,10 @@ NEWS/ChangeLog for hydroPSO
 
 
 0.1-56	11-Jun-2012
-        o 'hydroPSO'    :  -) the convergence of the algorithm controlled by 'reltol' is now defined by : reltol <= normalised swarm radius, 
-                              instead of reltol <= [ gbest(iter) / ( gbest(iter-1) +reltol ) ]
-                           -) minor change: 'gbest.fit.prior' is now initialized in 'gbest.fit' instead of 'gbest.fit + 10*reltol'
+        o 'hydroPSO'    : -) the convergence of the algorithm controlled by 'reltol' is now defined by : reltol <= normalised swarm radius, 
+                             instead of reltol <= [ gbest(iter) / ( gbest(iter-1) +reltol ) ]
+                          -) minor change: 'gbest.fit.prior' is now initialized in 'gbest.fit' instead of 'gbest.fit + 10*reltol'
+        o 'plot_results': -) 'drty.out' is now checked for existence before reading the results.
         
 
 0.1-55	11-May-2012
diff --git a/R/plot_results.R b/R/plot_results.R
index 70e4ec76c7fab04ee9572cb57bdc8e86541906d9..0eb78dd5a385c9af371a42ec725892582d8e3c0b 100755
--- a/R/plot_results.R
+++ b/R/plot_results.R
@@ -9,7 +9,8 @@
 ################################################################################
 # Author : Mauricio Zambrano-Bigiarini & Rodrigo Rojas                         #  
 # Started: 10-Nov-2011,                                                        #
-# Updates: 13-Ene-2012 ; 15-Feb-2012 ; 21-Feb-2012 ; 09-Mar-2012 ; 23-Mar-2012 #        
+# Updates: 13-Ene-2012 ; 15-Feb-2012 ; 21-Feb-2012 ; 09-Mar-2012 ; 23-Mar-2012 #       
+#          11-Jun-2012 
 ################################################################################
 
 plot_results <- function(drty.out="PSO.out",
@@ -96,6 +97,10 @@ plot_results <- function(drty.out="PSO.out",
    # Full path to 'drty.out'
    if (basename(drty.out) == drty.out) 
      drty.out <- paste(getwd(), "/", drty.out, sep="")
+     
+   # Checking 'drty.out' if necessary
+   if ( !file.exists(drty.out) )
+     stop("Invalid argument: the directory '", drty.out, "' does not exist !")
 
    # PNG directory
    png.drty <- "pngs"