Index: lib/redmine/field_format.rb
===================================================================
--- lib/redmine/field_format.rb	(revision 16542)
+++ lib/redmine/field_format.rb	(working copy)
@@ -486,7 +486,7 @@
 
       def validate_single_value(custom_field, value, customized=nil)
         errs = super
-        errs << ::I18n.t('activerecord.errors.messages.not_a_number') unless value.to_s =~ /^[+-]?\d+$/
+        errs << ::I18n.t('activerecord.errors.messages.not_a_number') unless value.to_s.strip =~ /^[+-]?\d+$/
         errs
       end
 
Index: test/unit/custom_field_test.rb
===================================================================
--- test/unit/custom_field_test.rb	(revision 16542)
+++ test/unit/custom_field_test.rb	(working copy)
@@ -204,6 +204,7 @@
     assert f.valid_field_value?('')
     assert !f.valid_field_value?(' ')
     assert f.valid_field_value?('123')
+    assert f.valid_field_value?(' 123 ')
     assert f.valid_field_value?('+123')
     assert f.valid_field_value?('-123')
     assert !f.valid_field_value?('6abc')
@@ -217,6 +218,7 @@
     assert f.valid_field_value?('')
     assert !f.valid_field_value?(' ')
     assert f.valid_field_value?('11.2')
+    assert f.valid_field_value?(' 11.2 ')
     assert f.valid_field_value?('-6.250')
     assert f.valid_field_value?('5')
     assert !f.valid_field_value?('6abc')