diff --git a/src/nvim/insexpand.c b/src/nvim/insexpand.c
index 3a87d77f89..ecbd9e2917 100644
--- a/src/nvim/insexpand.c
+++ b/src/nvim/insexpand.c
@@ -333,7 +333,6 @@ typedef struct cpt_source_T {
   uint64_t compl_start_tv;  ///< Timestamp when match collection starts
 } cpt_source_T;
 
-#define STARTCOL_NONE   -9
 /// Pointer to the array of completion sources
 static cpt_source_T *cpt_sources_array;
 /// Total number of completion sources specified in the 'cpt' option
@@ -4620,10 +4619,12 @@ static void prepare_cpt_compl_funcs(void)
         } else {
           startcol = -2;
         }
+      } else if (startcol < 0 || startcol > curwin->w_cursor.col) {
+        startcol = curwin->w_cursor.col;
       }
       cpt_sources_array[idx].cs_startcol = startcol;
     } else {
-      cpt_sources_array[idx].cs_startcol = STARTCOL_NONE;
+      cpt_sources_array[idx].cs_startcol = -3;
     }
 
     (void)copy_option_part(&p, IObuff, IOSIZE, ",");  // Advance p
@@ -6379,14 +6380,14 @@ static void cpt_compl_refresh(void)
           } else {
             startcol = -2;
           }
+        } else if (startcol < 0 || startcol > curwin->w_cursor.col) {
+          startcol = curwin->w_cursor.col;
         }
         cpt_sources_array[cpt_sources_index].cs_startcol = startcol;
         if (ret == OK) {
           compl_source_start_timer(cpt_sources_index);
           get_cpt_func_completion_matches(cb);
         }
-      } else {
-        cpt_sources_array[cpt_sources_index].cs_startcol = STARTCOL_NONE;
       }
     }
 
diff --git a/test/old/testdir/test_ins_complete.vim b/test/old/testdir/test_ins_complete.vim
index 9b69ce554f..c3de18446e 100644
--- a/test/old/testdir/test_ins_complete.vim
+++ b/test/old/testdir/test_ins_complete.vim
@@ -5524,4 +5524,32 @@ func Test_scriplocal_autoload_func()
   let &rtp = save_rtp
 endfunc
 
+" Issue #17907
+func Test_omni_start_invalid_col()
+  func OmniFunc(startcol, findstart, base)
+    if a:findstart
+      return a:startcol
+    else
+      return ['foo', 'foobar']
+    endif
+  endfunc
+
+  new
+  redraw  " need this to prevent NULL dereference in Nvim
+  set complete=o
+  set omnifunc=funcref('OmniFunc',\ [-1])
+  call setline(1, ['baz '])
+  call feedkeys("A\<C-N>\<Esc>0", 'tx!')
+  call assert_equal('baz foo', getline(1))
+
+  set omnifunc=funcref('OmniFunc',\ [1000])
+  call setline(1, ['bar '])
+  call feedkeys("A\<C-N>\<Esc>0", 'tx!')
+  call assert_equal('bar foo', getline(1))
+  bw!
+
+  delfunc OmniFunc
+  set omnifunc& complete&
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab nofoldenable