diff --git a/Meade.net/SetupDialogForm.cs b/Meade.net/SetupDialogForm.cs
index 6bf7613..36c8dac 100644
--- a/Meade.net/SetupDialogForm.cs
+++ b/Meade.net/SetupDialogForm.cs
@@ -5,7 +5,7 @@ using System.Windows.Forms;
 
 namespace ASCOM.Meade.net
 {
-    [ComVisible(false)]					// Form not registered for COM!
+    [ComVisible(false)] // Form not registered for COM!
     public partial class SetupDialogForm : Form
     {
         public SetupDialogForm()
@@ -33,14 +33,15 @@ namespace ASCOM.Meade.net
             {
                 MessageBox.Show(other.Message);
             }
-        }      
+        }
 
         public void SetProfile(ProfileProperties profileProperties)
         {
             chkTrace.Checked = profileProperties.TraceLogger;
             // set the list of com ports to those that are currently available
             comboBoxComPort.Items.Clear();
-            comboBoxComPort.Items.AddRange(System.IO.Ports.SerialPort.GetPortNames());      // use System.IO because it's static
+            comboBoxComPort.Items.AddRange(System.IO.Ports.SerialPort
+                .GetPortNames()); // use System.IO because it's static
             // select the current port if possible
             if (comboBoxComPort.Items.Contains(profileProperties.ComPort))
             {
@@ -56,7 +57,7 @@ namespace ASCOM.Meade.net
             {
                 TraceLogger = chkTrace.Checked,
                 ComPort = comboBoxComPort.SelectedItem.ToString(),
-                GuideRateArcSecondsPerSecond = double.Parse(txtGuideRate.Text)
+                GuideRateArcSecondsPerSecond = double.Parse(txtGuideRate.Text.Trim())
             };
 
             return profileProperties;
@@ -67,15 +68,38 @@ namespace ASCOM.Meade.net
             Activate();
         }
 
+        private bool _guideRateValid = true;
+
         private void TextBox1_TextChanged(object sender, EventArgs e)
         {
-          //const double SIDRATE = 0.9972695677; //synodic/solar seconds per sidereal second
-          var newGuideRate = double.Parse(txtGuideRate.Text);
+            //const double SIDRATE = 0.9972695677; //synodic/solar seconds per sidereal second
+            try
+            {
+                double newGuideRate = double.Parse(txtGuideRate.Text.Trim());
 
-          const double siderealArcSecondsPerSecond = 15.041;
-          var percentOfSideReal = (newGuideRate / siderealArcSecondsPerSecond * 100);
+                const double siderealArcSecondsPerSecond = 15.041;
+                var percentOfSideReal = (newGuideRate / siderealArcSecondsPerSecond * 100);
 
-          lblPercentOfSiderealRate.Text = $"({percentOfSideReal:00.0}% of sidereal rate)";
+                lblPercentOfSiderealRate.Text = $"({percentOfSideReal:00.0}% of sidereal rate)";
+                _guideRateValid = true;
+            }
+            catch (Exception exception)
+            {
+                //Surpressing this exception as if the value is not valid then it's not useful.
+                _guideRateValid = false;
+            }
+
+            UpdateOKButton();
+        }
+
+        private void UpdateOKButton()
+        {
+            cmdOK.Enabled = _guideRateValid && (comboBoxComPort.SelectedItem != null);
+        }
+
+        private void ComboBoxComPort_SelectedValueChanged(object sender, EventArgs e)
+        {
+            UpdateOKButton();
         }
     }
 }
\ No newline at end of file
diff --git a/Meade.net/SetupDialogForm.designer.cs b/Meade.net/SetupDialogForm.designer.cs
index b09f711..a714e67 100644
--- a/Meade.net/SetupDialogForm.designer.cs
+++ b/Meade.net/SetupDialogForm.designer.cs
@@ -89,6 +89,7 @@ namespace ASCOM.Meade.net
             this.comboBoxComPort.FormattingEnabled = true;
             resources.ApplyResources(this.comboBoxComPort, "comboBoxComPort");
             this.comboBoxComPort.Name = "comboBoxComPort";
+            this.comboBoxComPort.SelectedValueChanged += new System.EventHandler(this.ComboBoxComPort_SelectedValueChanged);
             // 
             // label3
             //